|“||Kujo. All the time, you were simply using me. And then you took my feeling for you. . .and crushed them||„|
|~ Billy thinking that Leesa betray his feeling|
He is close friend of Graham Aker and Leesa Kujo (who known as Sumeragi Lee Noriega in Celestial Being side), Lover and later husband of Mina Carmine (the DNA origin of Nena Trinity) and nephew of Homer Katagiri, commander of A-Laws.
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 is Homer Katagiri 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.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.
Even his relationship with Leesa already reconciled, they not continue their relationship into lovers. In Mobile Suit Gundam 00 The Movie: A wakening of the Trailblazer, its shown he have romantic relationship with the flirtatious female scientist named Mina Carmine. In epilogue mention he marry her and have a children.