This refers to characters who are typically cold and temperamental but affectionate when around the person they like, but even I’m not really sure. I think Chikagi is simply a violent person. By the way, a certain mushroom person [Kinoko Nasu] apparently said that Tohsaka Rin also doesn’t count as a tsundere. This makes the term even harder to understand.

HibiChika Complete Capture Book: Mahou Tsukai no Hako Small Encylopedia