Rubeus, also known as Crimson Rubeus (紅のルベウスKurenai no Rubeusu), is the first member of the Black Moon Clan to appear in the 20th century. He is charged with finding a person codenamed "Rabbit", who had fled into the Tokyo of the past. Besides following the orders of Prince Demand and Wiseman, he leads his own set of minions, the Negamoon Sisters.
In the anime, he is the love interest to Catzi, the oldest, though the majority of the sisters are in love with him. Only Prizma doesn't care for him. However, he does not return her feelings, and doesn't even care about her. At one point, he even states to Wiseman (who warned him something bad will happen to Catzi) that she is a subordinate.
Rubeus and the Negamoon Sisters were sent back in time to capture Rini and the Imperium Silver Crystal, while receiving instructions from Wiseman. Rubeus sent the Negamoon Sisters on each mission. At one point all five of them faced off against the Sailor Scouts, with Rubeus fighting Sailor Moon, who defeated him with Moon Scepter Elimination, although he survived.
Following the Negamoon Sisters' reform, Rubeus kidnaps the Sailor Scouts and blackmails Sailor Moon into bringing the Imperium Silver Crystal in exchange for her friends. Sailor Moon comes to rescue them, and Rubeus takes her and Rini aboard his spaceship. A battle ensues for the custody of the Sailor Scouts. Rini destroys the crystal that controls his spaceship causing it to catch fire. After Sailor Moon attacks Rubeus, the Sailor Scouts are freed and they, Rini, and Sailor Moon teleport away, leaving Rubeus onboard his burning ship. Rubeus is confronted by Emerald and begs her to take him back to the future. Emerald refuses and leaves Rubeus in the unstable spaceship saying he's failed too many times to be of any more use. The spaceship explodes and Rubeus dies. In the end, his obsession with killing Rini is his downfall. Emerald reports to Prince Diamond and Wiseman that Rubeus was killed in battle, but never mentioned that she left him to die.
His end in the anime is an ironic twist, as he once used the Negamoon Sisters as expendable pawns, while in the end he was revealed to be nothing more than a pawn himself.
In the manga, he kidnaps Sailor Mars, Sailor Mercury, and Sailor Jupiter but fails to capture Sailor Venus. He is loyal to Prince Demand, appearing with him to taunt the captured Sailor Moon. He realizes that there is more to Wiseman than what he sees and he is killed by Wiseman when he confronts him.