Billy Katagiri
(ビリー・カタギリ, Birī Katagiri?) is a fictional character in Mobile Suit Gundam 00. Billy is an MS engineer for the Union's MSWΛD in Season 1. In season 2, he becomes A-Laws' chief technical engineer to develop new generation mobile weapons for the ESF and the Innovators.

Billy is overall a sentimental, passionate, romantic, and caring person for those important in his life. After learning Leesa (Sumeragi Lee Noriega) is part of Celestial Being, his emotions had taken a negative change. He became spiteful, vengeful, and discontent how things turned out between him and Sumeragi. He has since poured all his feelings into MS development to help cope with his anger/hatred against Celestial Being. Sumeragi tells him that he's wrong and the A-Laws were wrong. Billy realizes he's wrong, his uncle is wrong.

By the end of AD 2312 (season 2), after reconciling with Leesa, he reverted back to his original personality and continues to devote his life into MS advancement.

He left the ALaws, however, his uncle committed seppuku

Billy is Homer's nephew. Though they have little interaction with each other due to their respective jobs, it can be said that the two hold a great deal of respect for each other.

Billy may not care his uncle is dead.