"THERE ARE NO MONSTERS ON THE FIRST FLOOR OF THE DUNGEON!!" - Dr. Tiff, hinting at the existence of the Berserker.
To find the Berserker, you have two options:
The first is to be level three or higher and explore the first floor of the dungeon. There's low odds for a lone enemy to spawn here which you must use Suppression on three times in one battle in order to calm the rage.
The other, as listed above, is seeing a scene related to Dr. Tiff scolding some students who claims they were attacked on the first floor of the dungeon (though if you ran into the Berserker and she fled, then visit Dr. Tiff's room, this scene is guaranteed to play out).
Regardless of what route you choose, here's how this route plays out.
Note that this is just a general outline for the route. It is not a walkthrough.
Finding the Berserker (Dr. Tiff Route)
The Protagonist visits Dr. Tiff and overhears her scolding two students, claiming they were attacked in the dungeon on the first floor. Dr. Tiff tells them to stop lying - there's no monsters on the first floor of the dungeon. The students tries explaining that it looked like a "vampire," having red eyes and being crazy.
Dr. Tiff ignores them but the Protagonist thinks it might be the truth.
From here, the Protagonist has two options: either heading to the library to speak with Julia, or head straight into the dungeon.
If the Protagonist decides to visit Julia, the Protagonist explains that s/he's heard about a girl on the first floor. Julia, if she's your slave, says there's something she needs to verify in person. Otherwise, she claims that the girl is out of her effective range. Regardless, she asks to go with the Protagonist to the dungeon. The Protagonist remarks that she said she wished to avoid the dungeon, causing Julia to explain that first floor is tolerable. The Protagonist agrees to bring her with him/her to find the Berserker.
On the first floor of the dungeon, they soon run into the Berserker. Julia tries reasoning with her, but is punched in the eye as a result. A fight erupts, during which the Protagonist and friends either uses Suppression three times, or defeats the Berserker, causing her to run off.
If the latter happens, Julia becomes possible to enslave - or heal up. If you enslave her, she'll explain that the girl is called Ashley and that she's aware of who Ashley is, but won't go into much detail. She just says she's had her selfish reasons for wanting to find Ashley but is willing to assist the Protagonist in enslaving Ashley, if s/he so desires.
Otherwise, the Protagonist soon runs into Ashley again. After using Suppression three times, Ashley asks what they're doing. The Protagonist explains that there's no danger anymore and she doesn't have to fight, but Ashley won't listen. However, she's forcibly calmed down by the Protagonist and tags along.
The Protagonist explains that "there's nothing beneath her hatred," saying that she might as well be empty. Julia is shocked and says this can't be right, but doesn't understand how it happened.
Finding the Berserker (Encountered Route)
While exploring the first floor of the dungeon, the Protagonist soon runs into the Berserker. If defeated, she simply runs off unless the Protagonist uses Suppression three times in the same battle.
Once done, the Berserker asks what the Protagonist is doing to her, to which the Protagonist explains s/he is simply trying to calm her down.
When asked what her name is, she says she doesn't have one - that having a name is stupid.
They head to the library and meet with Julia, who is shocked at seeing the Berserker and quickly examines her. She soon comes to the conclusion that the Berserker lost her mind - she's almost feral - due to using a Berserk spell.
Julia explains that, first of all, she must never use the Berserk spell again and asks if the girl has a name. The Protagonist says no and agrees to leave her with Julia.
Finding the Berserker (Submissive Route)
If the Protagonist is Julia's slave and they overhear Dr. Tiff talking or running into Ashley in the dungeon, Julia will tell the Protagonist to bring her to the dungeon. The Protagonist replies with "Yes, Mistress". In the fight that follows with Ashley, Julia orders the others not to do anything while the Protagonist casts Suppression on Ashley to suppress her rage. Heading back to the library, Julia will explain that they need to treat her as a person and says that her name is Ashley. The Protagonist promises to help out however s/he can.
Joining the Team
In the streets, the Berserker is waiting and says she's ready to tag along with the Protagonist. The Protagonist asks if it's fine with Julia, causing her to say that it's fine. She introduces herself as Ashley and the Protagonist welcomes her to the team, causing her to say "...Th-Thank... you."
Heading back to the library after reaching floor ten or later in the dungeon causes a scene to play out in which Julia is trying to teach Ashley how to read again, but Ashley says she doesn't want to - that books are stupid and that she wants to go back to the Protagonist. Julia yells at her, saying that "[Ashley] doesn't know what [Ashley] wants". This results in Ashley hitting her and the Protagonist heads over to Julia, who says it's nothing major.
If asked how Julia is, she says she's fine (+Affection) and the Protagonist remarks s/he never heard Julia lose her composure before. Julia explains that ignorance upsets her and will try not to lose her composure in the future.
If asked when Ashley will be ready (-Affection), Julia just states that Ashley isn't ready yet.
If the Protagonist is Julia's slave, the Protagonist will automatically ask how Julia is, who at first claims she's fine, but then admits to not being fine - she wants to fix her sister, but has no idea how she's going to do that.
Sending Ashley Home
The next time the Protagonist meets with Julia in the library, the Protagonist asks if Julia knows where Ashley is.
If enslaved, she says she doesn't know and the Protagonist will continue looking for her. Julia says that the Protagonist won't have to - she sent Ashley back home to her parents as Ashley is Julia's big sister. However, she refuses to explain what exactly happened. The Protagonist can either forgive her, causing him/her to say that s/he hopes that Ashley's recovery goes well. The Protagonist heads on home, only to find Ashley in his/her room, causing the Protagonist to remark that she was meant to go to her actual home, but Ashley considers this her home - the Protagonist beat her, so she wishes to stay here.
Otherwise, you can punish Julia, enslaving Ashley and breaking her mind in the process. This locks you out of the rest of this plotline.
If she's not enslaved, nor dating her, she will be reluctant in admitting to sending her back home. The same scenes as above plays out.
If you're dating Julia, she will openly admit to sending Ashley back home. The Protagonist can either say it's a bad idea, causing Julia to say that it might change her parents mind about allowing Julia into the dungeon. The Protagonist says that it's not like she needs a note from her parents to go in there, but she explains she still lives with them. The Protagonist scolds her, saying that if Ashley can't recognize her own sister, how happy will their parents be? She realizes her mistake and grabs Ashley.
If you say it's a good idea, she'll get surprised and asks why s/he thinks that. The Protagonist explains that they can see her physical body is well, even if her mind isn't. Julia panics and hurries off, getting Ashley back home again.
If the Protagonist is Julia's slave, the Protagonist says that it's a good idea, causing Julia to realize that it is, in fact, a horrible idea. She then brings Ashley back to the others.
The Mind Berserker
The Protagonist is requested to help Julia out - there's something actively blocking her attempts at fixing Ashley's mind, but she doesn't know what. As such, she requests that Jane helps them. The Protagonist can either agree, causing Julia some joy or asks for more information. This causes her to explain further about the problem.
Otherwise, you can refuse to help her. Julia asks why, causing the Protagonist to say that s/he likes Ashley like this, but won't stop Julia from trying to fix her mind. This locks you out of the rest of the plotline.
If it's decided to help Julia out, the Protagonist will speak to Jane for a bit. If Jane is your slave, she'll join without issue. Otherwise, she'll explain that unless it's something a staff did, she can't do anything to help but the Protagonist manages to convince her to help them anyway - in return for helping her in the future. The Protagonist seems a bit hesitant but won't say anything further about it.
Once back with Julia, Jane channels the spell and says that Ashley's mind is surprisingly narrow - from what she can tell, they're already at the center of it.
A fight erupts against the Berserker again. Once they win, Jane pulls them back out.
If Jane isn't your slave, she'll ask the Protagonist to come see her later - she has something in mind for him/her. If she is, she simply says she has to hurry on back to her class.
Julia asks how Ashley is feeling and she says she's feeling funny - Julia was hoping for Ashley to snap right back to her old self again, but realizes she's too optimistic. However, she plans on doing everything she can in order to get her sister back to normal.
Depending on the Protagonist's relationship to Jane, it affects what the Protagonist has to do next.
If the Protagonist has enslaved Jane, s/he simply needs to have reached floor fourteen of the dungeon.
If Jane is not the Protagonist's slave, the Protagonist has to speak to Jane in her classroom for a short scene in which she presents a magical contract. The choices made during the scene does not affect the overall plot of this route, but it affects Jane's trust and affection for the Protagonist.
Ashley's "Recovery"
Upon leaving the Protagonist's house, Julia runs into him/her and asks him/her to tag along. The Protagonist is confused as to why but agrees regardless.
Shortly after, Julia asks Ashley who [Julia] is. Ashley, vaguely, answers that she's Julia and that she's Ashley's little sister. Julia, looking stressed out, says that Ashley is fixed and that she finally has her sister back. The Protagonist, Val or Sarah (depending on who is present) remarks in various ways that Ashley is clearly not fixed. Julia says that she's fixed - however, the fast thinking process will take some more time.
If confronted, Julia admits to being sick of all the setbacks - she just wants her sister back. If the Protagonist is dating her, s/he can agree to stay with her or you can leave.
If the Protagonist is Julia's slave, the Protagonist automatically stays to cheer Julia up. The next scene depends on if you have Sarah's Mom or not.
Back in the school hallway, Sarah's mom says that she knows what's wrong with Ashley - she's missing her soul. The Protagonist asks if she's undead or a monster, but Sarah's mom says it's not quite right - she can't put her finger on it, but she suggests the Protagonist checks out the dungeon.
The Protagonist agrees to this and brings Ashley with him/her to the dungeon.
If the Protagonist does not have Sarah's mom, the above scene does not happen.
Ashley's Soul Search
If you have Sarah's mother, this route plays out.
Starting on floor twelve, the Protagonist tells Ashley to keep an eye out for her soul. Ashley quickly grows bored and demands to fight, but the Protagonist says that this is more important. She agrees to keep looking, but is quite unwilling in doing so.
On the thirteenth floor, the Protagonist remarks that there's been no sign of her soul. Ashley says she's bored and wants to fight, but the Protagonist tells her that if monsters attack, they will fight. Otherwise, they will continue search. Ashley agrees, but is reluctant in doing so.
On the fourteenth floor, the Protagonist comments on there being no sign of the soul and asks if something looks familiar. Ashley, however, just says that everything looks the same so the Protagonist admits to being too optimistic about it.
During the fifteenth floor, the Protagonist says feels that it's starting to get hopeless and wonders if the soul was on an earlier floor. Ashley says that she doesn't know and that she's bored, wanting to fight.
The Protagonist knows how much Ashley wants to fight, but they still need to continue searching. Ashley just says "Yes..." before suddenly feeling dizzy, causing the Protagonist to quickly examine her, saying that while she's not moving, she's still breathing at the very least. The Protagonist brings her back home to the Protagonist's house. If Molly is present, she worries because she can't tell what's wrong with Ashley and her magic doesn't seem to work on her.
If the Protagonist did not have Sarah's mom explain what happened, simply waiting on floor fifteen of the dungeon for a few minutes causes the same scene to play out, without having to see the previous scenes.
Ashley's Recovery
Back at the house, if Sarah is present, she will say she doesn't understand what happened to Ashley. Molly, likewise, will apologize for being unable to do anything.
Suddenly, Julia comes storming in and demands to see Ashley. Upon seeing Ashley, she asks what the Protagonist did, but the Protagonist says s/he didn't do anything: she just fainted in the dungeon!
Julia breaks down into tears and just wants her sister back. Ashley, suddenly, speaks up - she asks Julia to get off her as [Julia] will bruise [Ashley's] boob otherwise.
Julia is shocked upon seeing this - by all rights, she shouldn't have been fixed. She quickly asks what she remembers, causing Ashley to admit she doesn't remember much - her memories are hazy from that she used her Berserk spell until that she was enslaved by the Protagonist.
The Protagonist corrects her - s/he didn't enslave Ashley, s/he just used Suppression to suppress her rage. Ashley remarks that they can't argue with the results and is prepared to become the Protagonist's slave for real. Julia tries to stop her, causing a choice for the Protagonist: enslaving Ashley or freeing her.
Freeing her makes the Protagonist say that s/he made a deal with Julia to fix Ashley's mind and was planning on setting her free. Both of them thanks the Protagonist and Julia says that Ashley can go back home.
However, Ashley says she can't - she haven't graduated yet, so the odds are that their parents will just keep the locked in their house to avoid her dying for real. Julia is shocked and tries to reason with her, but with no success. Instead, Julia agrees to tag along to the dungeon - even though she's normally unwilling in doing so, she says she won't mind as she wants to keep an eye on Ashley to make sure she doesn't disappear on them again.
The Protagonist admits to not being sure if s/he can always keep both of them on the team, but promises to at the very least try to keep them with each other.
Ashley agrees to this and asks if they mind if she takes a shower - she's been wearing the same clothes for a year, so they're likely gonna fall apart any second now. The Protagonist says it's fine and welcomes her to the team - for real, this time.
If you decide to enslave Ashley, Ashley doesn't attempt to resist it and instead is willingly becoming the Protagonist's slave (note: she does NOT count as a Willing Slave on this route).
Julia, shocked, asks the Protagonist to enslave Julia as well, saying she'll tell everyone what they've done otherwise. Should the Protagonist refuse, Julia is saddened about it but decides to accept this fact.
Otherwise, if the Protagonist agrees to enslave Julia, Ashley quickly agrees to this, saying that they can be slave sisters for real. Julia is then enslaved and a scene plays out, in which they make out with each other in front of their new Master/Mistress.
If she's not enslaved, Julia just asks to be allowed to be brought to the dungeon in order to keep an eye on Ashley. The Protagonist agrees to this and tells Ashley she's welcome to the harem, for real this time.
Should the Protagonist be Julia's slave, Julia will order the Protagonist to release Ashley, who does so willingly. Ashley becomes curious about why the Protagonist refers to Julia as "Mistress," causing them to briefly explain their relationship. Ashley instantly gets hooked on it and says she should be a mistress-by-proxy as she's Julia's big sister, something Julia doesn't seem to keen on agreeing with. However, Ashley is now free from the Berserk spell and, to make sure she won't use it again, as long as the Protagonist remains Julia's slave, if Ashley is added to the party, Julia will automatically fill the other party slot. It's still possible to bring Julia and one other person as normal, however.
Finally free of the Berserk spell, Ashley finally gets to live her life as a regular student again.