User blog comment:B214/High School DxD EX/@comment-26535460-20151128041730/@comment-27232228-20151203150959

Considering that Airi did a ballet-like spinning move to activate her CB, I go with ッ (the move spells 鬼 子 化 ツツ, "Oni ko ka tsutsu"), plus, the printed form difference is the lines, "tsu" is more slanted, while "si" is a bit straight).