Despite his massive physical and magical talent, Yakumo has a childish personality with an inclination to play childish pranks. He constantly hides his presence and sneaks up on people to scare them. Just looking at him gives a sense of a lack of propriety. He likes darkening rooms and spooking people that come in. He has an "altered" sense of social boundaries, somewhat like Tatsuya, readily admitting to gathering intelligence about people. He also likes teasing people, which some don't really take well. He is a friendly guy who helps out upon request, although this might be only for Tatsuya, as he was once called Yakumo's favorite disciple by Ono Haruka. However, Yakumo has also given Haruka high praise for her abilities, according to Fujibayashi Kyouko. But even that has its boundaries, as shown when he wouldn't teach Tatsuya, officially, a way to counter Ghost Walker, because he wasn't a student of him at that moment.