Raijinmaru is actually female, contrary to Tamakoma members' beliefs.
"Raijin" (雷神,"Raijin"?) is the Shintō god of lightning, thunder, and storms. "Maru" (丸,"Maru"?), meaning "perfection" or "full circle", is a common suffix for Japanese male names (despite the fact that Raijinmaru is a female).
Raijinmaru's name comes from the dog of Yōtarō's favourite television hero.
According to Border Briefing File, Raijinmaru is both Border's most popular and the least concerned with popularity.
Kazama has been caught on camera attempting to ride, or at least get on, Raijinmaru.