Katie Bellinger was the daughter of inmate Shirley Bellinger.
In her mother's crime flashback, she is seen sitting in the back seat of the car. After waiting at a stop light, Shirley proceeds to drive their car into a lake. Minutes later, Shirley emerges from the lake having escaped the sinking car, but Katie is confirmed to have died of drowning.
Shirley is charged and sentenced to death for murdering her daughter and becomes the first female prisoner ever in the Oswald Maximum Security Facility. When asked why she killed Katie, Shirley says that it was an accident but that it had to happen regardless.
In Season 3, Bellinger tells Sister Peter Marie Reimondo that while Katie was a joy she witnessed orbs around Katie and saw her levitate a plate. Convinced that Katie was possessed by the devil, Bellinger justified killing her in order to save her.
In Season 4, when Bellinger's ex-husband visits Bellinger, Bellinger tells Zeke that Katie is not his daughter but really his half-sister. Bellinger claims that Zeke's father raped her and Katie was conceived of that rape.