Mugen is Rei's partner. He is actually a card, Mugendra. He doesn't seem to like Ian, though they can get along.

He met Rei sometime after he acquired Ultimate-Siegfried and Ultmate-Odin, and thus doesn't know about Rei's past.

Mugen's love for Magical Star led him to purchasing a very expensive figure of her. He spent all of the crew's money, so they decided to look for work to earn more. They ended up becoming bodyguards for Magical Star's first ribbon, which was actually stolen by the person who currently had it. After getting the ribbon back, Mugen found out that Magical Star would actually be on the planet where they were.

Mugen planned to return the ribbon to Magical Star, but ran into Watari, literally. The bag carrying the ribbon was damaged, which got him mad. After learning that Watari and the rest of the Triumvirate had kidnapped Magical Star, Mugen was furious, and wanted to battle. He couldn't, so Rei battled in his place, and won. However, he sacrificed Mugendragon to summon Ultimate-Kingtaurus, so Mugen couldn't even give the finishing blow. However, he was able to return the ribbon in the end, which made Magical happy.

When Rei recieved Ultimate-Siegwurm-Nova, it seemed to have a strange effect on Mugen. However, he was cured of it once Rei summoned him in a battle.

Mugen eventually evolved all the way to an ultimate form, Ultimate-Mugendragon. He dealt the final blow in Rei's battle against Nakes.

(Source: Battle Spirits Wiki)