Rei's an oni, and a pretty striking one at that—though his main passion in life is honestly just eating. He can transform into an "ushitora," this massive hybrid creature that's basically a tiger-bull fusion, which tends to happen when he gets really angry. His Japanese is still pretty rough; he usually manages only a handful of words at a time.
Back in the day, both Ran and Lum had crushes on him, but he ended up getting engaged to Lum instead. That didn't go over well with Ran, who felt betrayed enough to develop a serious grudge against Lum. Eventually Lum broke things off with him, but Rei wasn't ready to let her go—he followed her all the way to Earth to try winning her back. While he was there, watching her with Ataru made him incredibly jealous, and whenever he got worked up about it, he'd transform into his ushitora form.
Content compiled by AnimeList.moe from publicly available sources.
