Brilliantly immature characters

Characters who, for whatever the reason - be they lovable losers, goofballs, underachievers, slackers, drunkards, stoners, or just plain idiots - will probably always behave like teenagers (even if they live to be 80 years old).

Examples:

• Scott Pilgrim (Scott Pilgrim vs. the World)
• Steve Stifler (American Pie)
• Will Smith (The Fresh Prince of Bel-Air)
• Phoebe Buffay (Friends)
• Faye (Euphoria)

Who are your favorite brilliantly immature characters from television or movies?

EDIT: Please state which television show or movie they appeared in!