During the St. Michael's arc after Tsuzuki had killed Sargantanas/Saagatanasu from The Devil's Trill, it's mentioned by Tsukiori that due to Klingon Promotion, Tsuzuki is the most qualified for the position of Demon Brigadier even if it's unheard of for a human to become a commander in the demon empire. Their best explanation is it's because Grand Duke Ashtaroth cares more about power than anything else. Fast forward to the Kyoto arc, and The Reveal about Tsuzuki.
A very subtle example: why is Hisoka so intolerant of heat? Because he spent the majority of his life in a cellar, underground. Underground spaces are considerably cooler than above ground...
Tsuzuki's attraction to Hisae could very well be based on a past incestuous relationship with his sister. He makes the statement that Hisae should "dance like she wants/loves him," if they're to dance well together. When asked who taught him how to ballroom dance... he claims it was his sister. (Made more suspect by him later speaking of sharing souls.) Comparing Hisae's design to Luka's... they're very similar.
.) Comparing Hisae's