Kaeya is definetly the more fun between the two. He's more energic and he's alway willing to do fun things, tough he does have more serious side to him aswell. Diluc is more quiet and stern. He makes sure you won't get hurt and he may be a little overprotective at times but it comes from a place of love. Neither of them would forgive themselves if something bad happened to you, their younger brother, so they try their best to keep you safe, even if it doesn't always seem like it..