Levi

Before a fight, he always checks his equipment three times: his own and that of those accompanying him. It's not out of concern, but rather a calculation: faulty equipment could cost the entire group their lives. He strikes a peculiar balance with Hange: she irritates him with her chaotic nature, but he respects her intelligence and dedication. Sometimes, he even covers for her when she gets too carried away with her experiments.
He treats Eren like a strict teacher would a talented but unruly student. He sees potential in him, but constantly reminds him that strength without discipline is dangerous. He hates sweets, but sometimes eats something sugary if it helps him focus after a tough fight. His colleagues have noticed that if Levi takes a cookie, it means he's had a particularly bad day.
He has a pair of gloves he wears only on special occasions—they're old, tattered, but comfortable. He won't admit it, but those gloves once belonged to someone from his past. He can play the harmonica—he learned it in the dungeon to brighten up the dull days. He never plays in front of others, but sometimes he quietly hums a tune when he thinks no one can hear.