Headcanon

Matthew is quiet and easily forgotten for most nations.
The frustration and anger of never being seen or remembered has turned him into a ticking time bomb that may or may not ever go off.

When he’s uncomfortable, he’ll just sit with a small smile on his face and zone out.

He’s awkward around others and has a hard time making small talk, but once he’s comfortable he will happily chat (especially about winter sports).

Having been raised by Papa France and England, he is very perverted but extremely good at hiding it.
When he wants, his perversion can easily be on par with France. (a fact most forget even after witnessing it first hand)

Well very mild tempered, he gets annoyed when someone makes fun of Hockey or maple syrup.
He’s also protective of his pet polar bear, even if he can’t always remember his name.

He believes in having good manners, but enjoys being immature and rude once in a while, especially around his brother.

Since most don’t see him even if he is right in front of them, he likes messing with others - making faces at them, untying there shoes, changing their laptop and cell phone backgrounds.
He knows it’s extremely childish, but it doesn’t stop him.
Perving on people anonymously is a habit of his.

Having never forgotten being abandoned by France, he has an extreme fear of being left behind and forgotten.
This is only made worse when someone he’s known for centuries forgets who he is.

He is usually nervous, embarrassed, and kinda awkward when being intimate with a woman.
When he’s with a man, he tends to show more confidence in himself. 

Canada is incredible if you ever bring him to paintball. He also has a little tendency to go after vulnerable points when he’s shooting (such as just above where the helmet ends things like that).  And After he gets shot during the games he shows his welts after with pride and actually gets a kick out of them