He was a part of a team of meisters and weapons (among them Stein and Death Scythe) who were responsible for the annihilation of the Star Clan. He apparently knew White☆Star and saw him as he killed an innocent, devolving his clan into madness and becoming a Kishin. After the annihilation of the clan, it was Sid who took Black☆Star, White☆Star's son, disregarding Stein's advice to either kill or abandon the baby, and raised him at DWMA. As such, he is considered not only Black☆Star's mentor, but flashbacks have shown that they shared a brother/brother or father/son relationship.