Ultra Magnus (Robots in Disguise)
Ultra Magnus was Optimus Prime's "brother," who was passed over by Alpha Trion in favor of Prime for receiving the Matrix. Jealous and angry, Magnus left Cybertron. Arriving on Earth years later, he resolved to steal the Matrix from Prime by force, attacking his brother. Eventually, he pretended to forgive him, but used the handshake that followed to try to drain off Prime's Matrix energy. The unexpected result was that they combined into a super-robot known as Omega Prime.
Magnus retained some Matrix energy after the merger ended, using it to give the Car Brothers upgraded "super" forms. Eventually, he began to help the Autobots on occasion, even forming Omega Prime a few more times. In the end, as Omega Prime, Prime and Magnus finally defeated Galvatron.