Jo Donner does not yet have a known codename.
Jo was born Joni Wilson, Chad Wilson's younger sister. Their family was very religious and very anti-mutant. Their elder brother, Mark, was killed by their uncle when he manifested, though the official fiction was that he ran away. Joni and Chad tried to protect each other, as their father was an alchoholic and getting more and more abusive. Chad was a very good hacker, and one night he manifested as a mutant with power over computers while he was drugged on painkillers, and stopped an attack by the supervillain called the Palm. The Palm responded to this by bombing their house. Both their parents were killed, but Chad was in the hospital due to complications of his mutation and Joni was at a friend's house. Soon after, Chad was found dead as well, and Joni was given into the care of her aunt and uncle, along with a large inheritance that had been in Chad's account, which was put into trust for her by the bank.
Joni did not believe that her brother was dead, though it is unclear whether this was denial or if she really knew something. Her aunt and uncle spent the income from her trust fund on themselves, and she engaged in some risky behaviors. At some point during this period she manifested as a mutant as well. She then ran away to a church that is part of an 'underground railroad' for mutants, whose pastor, Reverend Carstairs, is the Scarlet Knight. Sara Waite and her father got Joni out of the church just before an attack by her uncle and his Humanity First! group, and took her to the Medawihla reservation to be reunited with her brother, who had since become twin sisters, Paige and Petra. Eloise and Ben Donner then adopted all three girls, and sent Joni, under the name Josephine or Jo, to Whateley along with Paige, while Petra went to Rome.
Jo tends to be very aware of what is happening around her, and very protective of her siblings. She is also very athletic, and likes to skateboard. She was a gymnast, with the goal of competing in the Olympics, before manifesting as a mutant disqualified her.
She is a Serial Linear Precog, meaning she sees the future about 30 seconds to a minute ahead of time. She has been seen to use this while skateboarding, to allow her to make extremely risky moves without getting hurt, and to know what a person she is talking to is about to say, and responding to it without them actually having said it aloud. This makes conversations with her very interesting. Jo may also be an empath.
It isn't clear which cottage she is in. In Merry 7, she is in Dickinson. However, in Stress Fracture, Fractious talks about her new roommate Jo, with enough details that it is likely to be the same person, and Fractious rooms in Whitman.