Despite her youthful appearance, she is actually hundreds of years old. Centuries ago, a meteorite fell into her branches, splitting her soul from her body. Her soul, appearing as a human form, can travel as she pleases, but is limited to the forest boundaries, falling into a coma-like state by taking only a few steps beyond the trees.
Slow, Sleepy, Spacey, Polite, Not too bright, Patient Link to the script
Just going through some short descriptions of her.
Hair: Platinum blonde with a hint of green, chest-length, bangs cover part of one eye
Eyes: Bright green
Skin tone: Light
Clothes: Milky brown kimono-like robe with minimal or no design
Summary: Though she can only speculate and make guesses about what happened, she came to the conclusion that her soul was split from her body when a meteorite collided with her hundreds of years ago. Rather than granting the wish of a sentient being, her soul became sentient and was split from her body in a humanlike form. She likes to learn, though her knowledge is extremely limited, due to the fact that she cannot leave the forest. Lately, even her stone-like patience has begun to waver, and she has been testing the limits of her soul by stepping farther from her body each day.
I think she should be the only girl that doesn't already have a name. The protag and her will have a conversation after they've met a few times and he'll ask what her name is, only for her to say she doesn't have one. Then he'll take a moment to think and say Ashe because she's an ash tree (so clever, I know).
Human Arc (Only unlocked after seeing the Part 2 good ends for all the girls)
School has begun, and somehow you've managed to fill out all the paperwork so that she can enroll at school with you. The two of you are now living together since she has nowhere to stay.
Adapting to human life is the easiest for her out of all the girls, but she dislikes how fast-paced everything seems. Since her life used to be more or less infinite, she could take her time doing whatever she wanted.
Conflict idea: A section of the forest will be cleared to expand the residential district. Her old tree body will be included in the section that is cleared, and, while it no longer holds any sort of spiritual connection to her, she doesn't want to let them dispose of it. Could be used as a source of conflict in Part 2 instead so that the issue seems more urgent.