|Ranged Weapon||EZ Katze I|
|Accessory Weapon||EZ Wolf I Ausf. F|
Billy "Leopard" Koller is a Shield class sinner, and one of the protagonist's companions.
Billy is a optimistic and energetic sinner who considers himself a stylish person, as he often like to brag about his hair to his fellow Sinners. He claims to have more knowledge about the Fraternity and the 'streets' than anyone else, and refers to himself as "Big Cat". He's particularly attached to Mattias, being very concerned with the Protagonist's relationship with his old friend and even more intent on impressing him at every chance he's given. He shows romantic interest in Beatrice as well, but claims she's too far out of his league and thinks its better for him to find someone from the 'streets' like him.
Billy is a rookie sinner and an acquaintance of the protagonist.
He doesn't have a role in the story outside of giving hints and opinions to the protagonist in the Hub and help them out on operations as a companion.
- He, Carlos and Shizuka are the only companions that the protagonist doesn't meet during the party hosted by Nina and Uwe.
- Billy is voiced by Seiichiro Yamashita
- He can be recreated in the Character Editor.
- He, alongside Nina, Kai, Shizuka, Sergio and Marie have no role in the story outside of being companions.