Dex-Starr was originally a normal house-cat, adopted by a kindly old woman who he shared a close bond with. After her death, he remained with her until being kicked out of her apartment by a group of movers. Abandoned on the streets, two thugs trapped him in a box and decided to drown him in the nearby river for fun. Filled with hatred of the human species and their treatment of him, a Red Lantern ring came to Dex-Starr, giving him the power to murder the two thugs who tried to kill him. He met up with the other Red Lanterns soon after these events, becoming somewhat of a pet to their leader, Atrocitus.
Lego Batman 3
To unlock Dex-Starr you must complete all of the Green Loontern quests on Ysmault, then complete a quest for Catwoman.