Twelve was one of two members of a terrorist group known as Sphinx, working with Nine as his primary partner. He and Nine grew up together in an institution for children known as the Settlement, and engaged in terrorist activities, with unknown motives.
On the outside, he's a typical jokester: light-humored and energetic, enjoying messing around in other people's business. However, he is fully capable and willing to adopt ruthless and violent methods if the situation calls for it. This contradictory set of personality traits are the result of the psychological damage sustained during his stay at the aforementioned children's institution. Nonetheless, he is a loyal and dependable friend, has a genius mind and a keen sense of social justice. He also has a compassionate side, as evidenced by his interactions with Lisa Mishima.
Twelve changes dramatically upon interacting with Lisa Mishima. When she runs away, he goes after her against Nine's wishes. He is made by Five to choose between saving Lisa and betraying Nine, and ultimately chooses to save Lisa. However, he later helps Nine out of a sticky situation at great risk to himself.
In the final episode, shortly after he, Nine and Lisa are reunited, he is shot and killed by the American government.