Rin Tōsaka (Fate/…)?

Don't think so. Source link seems to imply an original character. Reverse image search turns up https://danbooru.donmai.us/posts/4929383 which also states original work by the artist.

Can see why one might have thought it. Eye color is close, lighting on the hair suggests it is possible, but the pose with the teddy bear is out of character for Rin given the apparent age of the character in the picture. Even when younger in Fate/Zero, she was seriously studying mage-craft...