The Deceased Mage
Sarah's Mom is an unnamed NPC who only appears during a short period of time as of version 0.62.
During Sarah's Home plot, after you lose against the (more or less) unwinnable battle, Sarah's Mom will take posession of Sarah and use the Bubble spell in order to defeat the monster. This unlocks the Bubble spell for Sarah to use in combat as well.
Once defeated, she'll transport the Protagonist back to his/her house. There, she'll introduce herself and explain what happened. She asks the Protagonist to look after Sarah and is grateful that she got a chance to see her beloved daughter once more.
If the Protagonist chooses to use Suppression, it'll unlock the Possession spell and also an alternate scene when meeting with Sarah.
Regardless if you use Suppression or not, heading back to the lake will cause a scene to play out. You can either explain what happened, or introduce Sarah to her mother.
If you choose the former, she'll get overwhelmed and hurry off as she wishes to be alone for a bit and the scene ends.
If you choose to introduce them to each other, Sarah scolds her mother and asks why she left her an orphan. Sarah's mother gets surprised at this, remarking that Sarah's father should still be alive, as she haven't met him on the other side yet.
Sarah thus scolds her father, saying that he probably just headed off on some adventure without caring about her, but her mother corrects her, saying that her father loves her deeply, though she doesn't know why he didn't bring Sarah with him.
The scene ends with Sarah's mother saying that she loves Sarah deeply, causing Sarah to reply that she loves her mother, too.
Alternate Encounter
If the Protagonist is on the Submissive route, she will show up for a different scene, depending on who was KO'd in the battle and not.
Looks
In terms of looks, Sarah's mother looks the same as Sarah in her Eager to Please form, just without the heart shaped pupils.
Ashley's Soul Search
Late in the Berserker plotline, after Julia "fixes" Ashley's mind and the Protagonist leaves the library, Sarah's mother will appear and say that she thinks she know what's wrong with Ashley; Ashley is lacking her soul. She can't quite explain it, but suggests that might be the reason she's gone feral to begin with. The Protagonist takes this advice to heart and brings Ashley back into the dungeon in order to find Ashley's soul and restore her to normal.
Trivia
- She's one of two parents physically shown in-game, the other one being Angie.
- As of version 0.47, she is no longer needed to obtain Ashley's soul.
- Enslaving her does not shift you to the Tempted Hero route. However, it's possible using Possession on your party members will cause that in the future.