Serina is an A.I. closely related to Cortana, though it is unknown how. It is probably that, since she came into being some 30 years before, Serina is Cortana's Great great great grandmother (30 divided by 7, remembering that A.I's live for 7 years. So don't start having disgusting fantasies about Cortana).
Serina was, in a nutshell, a stuck up bitch compared to Cortana, and never got into a modified battle suit like Master Chief does (see Cortana). Because Serina sucks, she chose a clothed avatar. I mean, seriously, who the hell does that!? Anyway, I'm just probably pissed about not seeing Serina nude. Even though she never had sex, due to lack of Horny A.Is, she still had a memory of a kiss, leading her to masturbate when off-task. She got caught masturbating by Ellen Anders, who were going to masturbate herself. When Anders accessed Serina's CPU, she saw Serina was masturbating while thinking of Warthog Guy, who she was going to have for herself after she was finished, she began hating Serina ever since.