Mouse leaving in the end wasn't just him being a Humble Hero, he was terrified of his reaction to the Princess.
The "Holy", the most powerful of the KANNON clan that controls time. Xelpherpolis claims to have been bewitched and not originally intending any of this; and the other villains claim that "Master Rank" KANNON clan members always try to achieve the power of "Holy."- Of course, this could just be villains attempting to justify their actions; but even Claire, a "Master" Rank, started acting very differently once her powers started coming out. And that was just a month or so. What if his having "Soldier class" KANNON genetics meant his instincts were attracting him to Roleiyu; and not just Love at First Sight?
- It was Roleiyu's question "Do you want to rule my heart like Xelpherpolis did?" question that freaked him out. No matter his reaction, whether Roleiyu should be free of anyone associated with KANNON; (Mouse is a Nice Guy) or just a pragmatic worry that Power Corrupts literally.
