My vote goes to Sir Noi Crezant from Hoshi no Samidare. He's hilarious, and his banter with the main character provide huge entertainment in every chapter of Lucifer and the Biscuit Hammer. The drama moments at the end of the manga is very well depicted too, I guess he's my all time favorite for mascot characters.
I like Kon as well, he's so damn funny and his narcissistic attitude always makes me laugh
Evangelion new movie made me remember about the existence of a mascot character named Pen Pen. He provides an ice breaker even though very small in EVA, so I guess that he's one of my favorite mascot characters too.
Every demon who ends up going to the human world in Yondemasuyo Azazel-san. One of the things that make the manga and anime hilarious is that their real appearance is actually quite scary looking and yet they get constantly abused by Akutabe.
There's a whole bunch of demon characters, and my two favourites are Lucifer and Beelzebub.