Let's start with the facts. Erole speaks his mind, is not easily embarrassed, and likes to be in charge of situations. The latter is seen by his acting as a general when first arriving in Nalavale and leaving detailed instructions for Larf after he had gone, even though he hadn't been appointed by him. He has no problem insulting people (and laughing about it), or ignoring (or sometimes interrupting) them if he feels they're not saying anything important.
Erole is very confident in his abilities (and yes, that is an understatement), as evidenced by some of the things he says in the game. As a result, other characters view him as cheeky, which he is. He probably can't spell 'humble', as even when being made fun of, he finds a way to take it as a compliment. It's almost difficult to label him as conceited or cocky because he's so smooth and subtle about it.
He is very loyal to his queen Fals, and the kingdom of Gran. He also seems to respect royal authority (through probably not so much with generals). He travels to Nalavale to help Larf under the direction of Fals, and returns to Gran because of a national emergency, but only does so after gaining the approval of Larf.
I've read that in the Dreamcast version, Erole is described as eccentric. If you've played the game then you likely noticed how strange and complicated he can be. In a good way, of course. One character stated that he's 'a bit over the top'. This is proven by some of his battle tactics, which he finds humor in.
Erole is usually seen smiling or grinning, but can be serious when necessary.
Introvert or Extrovert?
He is not an introvert because he does not shy away from people or social situations. He also doesn't mind taking risks as far as his tactics are concerned. While this may cause some to label him as an extrovert, this isn't likely either. By his conversations, it's implied Erole that he doesn't like verbal conflicts. He also thinks before he speaks and acts, and focuses on important matters when the time calls for it. So, Erole is actually more of an ambivert.