She is mentioned as the mother of Zeke in the first two books, but she does not appear in the story until the third book.
In The Chestnut King, Tilly Johnson peeks out her door in the middle of the night to find Henry York Maccabee, Henrietta Willis, and Grandmother Anastasia in need of help. She helps them prepare for their trip to Endor, and even lets her son Zeke go along. She cares for Grandmother Anastasia while they are gone.
Her next appearance is of rushing through crowds of people, which she hates, to find a police officer. Her "mother", she says, won't wake up and is having trouble breathing. This, of course, is Grandmother Anastasia. She takes her to the hospital, but Grandmother doesn't make it. Tilly buried her in a small plot next to her husband, Timothy.
Caleb gives her a band with a blue gem, and they get married. Tilly and Caleb build a house next to the sea.