Being a bullied child has left Itsuki with a desire to be special and superior, his sense of justice and need for praise both stem from this, as does his inability to deal with the meek Rishia, who reminds him of his weakness. He can also be quite gullible and stupid, though not to Motoyasu's extent, as all of his teammates excluding Rishia are in fact extremely untrustworthy people who just praise him all the time.
He feels in his own mind that he's the strongest of the heroes, and he undergoes stealthy missions to kill corrupt nobles and save villages almost always just for the praise. He's extremely petty, and not very smart, believing Naofumi is guilty on both the false rape charge and later believing the Shield Hero stole credit for his achievements, which basically isn't hard for anyone to do because Itsuki pretends to be a robbin-hood-esk Hero who conceals his own activities often. He even believed it when his comrade framed Rishia for breaking an item of his which he was told, yet another deception by the way, had the power to raise his EXP intake. In short, he's stuck up, gullible, and wants to be praised by others. The one thing Naofumi has ever said to him that has genuinely enraged Itsuki is in fact as simple as "i thought you were better than this, my evaluation of you has dropped," to which he was willing to shoot the Shield Hero in the back of the head for it. He's even willing to conceal his power form allies and wait for them to actually be in danger before he'll step in so he can impress them.