As Kaito has a hard time thinking about this himself, I fielded the question to Hakuba. The response?
“Quirkiest. Quirky IS Kaito. Kaito IS quirky. What is the most Kaito thing that Kaito does? That’s tough. Is it meddling in others’ business? Is it taking bird baths? Perhaps it’s the hoops he’ll jump through for chocolate. Or the need to be in everything, at all times. Taking control of a room and getting to know everyone there and assigning himself as personal coach to each, complete with laundry list of what they need to do to fix their lives as far as he sees it. Or taking things apart just because he can and then put them back together like he’s an avian Mary Poppins.”
"Basically, he’s a Jack Russell Terrier… with wings.”
“…Excuse you.”
“Sorry, dear. It’s absolutely true and you know it.”