Only a few brief glimpses of her face are seen in the beginning and toward the ending when Wilbur takes Lewis back to his time. It's assumed that she's a redhead with curly red hair in a bun hairstyle and her attire was a dark, grimy cloak with red pointy shoes and her attire in Lewis' drawing was a dress, a bow in her hair and some flats.
Role in the film
She first appears in the beginning of the film, in 1995 abandoning her child at the Sixth Street Orphanage. She looks at Lewis and gives him one last hug before a loud scraping noise alerted her and she left him at the front door of the orphanage, walking away into the distance. Having disappeared, no one has seen her since and only Lewis has a very faint but important memory.
It's unknown why she had given Lewis up for adoption. Mildred tells Lewis that she thinks Lewis's biological mother maybe didn't know how to take care of him. From little that can be seen of Lewis's biological mother, she does appear rather young, so it's possible that at least part of the reason she gave her son up for adoption was that she didn't feel ready to be a parent and wanted to give Lewis a better life.
Her true name is unknown and her background information is unknown.
It's unknown if she had a spouse or if she was widowed, separated, divorced, or single. But given her apparently youthful appearance, it's possible that she was no older than her early 20s.
According to the DVD commentary of the film, her CG body model is the same as Franny's, just with a cloak.
When Lewis asked his future self (Cornelius), if he had ever met her, he answered nervously that Lewis would have to find out for himself, possibly indicating that he did find her at some point before the events of the movie.
A deleted scene that shows Lewis using the Memory Scanner on himself successfully instead of Dr. Krunklehorn would have shown him getting a glimpse of his mother, possibly before she gave him up. She appears to be crying at a table before coming over to see him. He turned off the Scanner after a moment, deciding it was enough for him.