His tattoo reads "MOTHER LOVE", which is also the name of a song 의해 Queen. It's progably a reference to the band as they've done it before with Claire in Code Veronica. (She wore a 재킷, 자 켓 with the words "Let me Live" on the back; another song 의해 queen.)
But getting back to Billy~ Personally, I think the lyrics in the song MOTHER 사랑 can relate to him.
