All the headcannons

America holds the fact that he beat Japan, the most dangerous nation at the time, and uses that to blackmail him.

Eren sleeps with a picture of his mom under his pillow. Mikasa found it necause the wind blew it in.

Japan had a human friend once as well. But his friend betrayed him in such a way he couldn't trust anyone anymore and decided it would be easier to bottle everything up than let someone break into his shell.