So, how did it batch of misfits get along as children ? Also, Itsuki as a child was basically like any other kid, maybe a bit more energetic and eager to help others, but other than that, as normal as a ghoul could be;and probably would take extra care to watch over his human sister since humans are so fragile. Nowadays, Itsuki's two steps away from lacking any feelings of remorse, unconsciously despises his siblings cause he believes they abandoned him, and in general, is not hesitant to kill almost anyone that gets in his way, including his siblings.
I had the idea that he may have tried to get in touch with the others when he and Hiyori got together, since he was finally feeling happy and wanted to bring his family back together.