And it's interesting that in almost all MK fic Kaito is "Kaito" but Saguru is "Hakuba."

Yeah, have wondered about that before.
No thanks to Ysabet or MOrgan or Po'e, it's beginning to look like this may end up another series. *sighs* Was so happy to have one WiP done too.
But I think it's because of personality. Hakuba's personality just so distant, it feels proper to use his last name instead of his first.
From what I've been reading about both the British and Japanese school systems, it would be the norm to use their last names instead of their first names as well.
But with 'Sa-chan's Mum running around, plus 'Uncle Erotica', don't be too surprised if 'Saguru' begins to replace 'Hakuba'.