So I've had an idea for a character subclass, (or maybe a class?) that sounds really fun. Basically, the character is kind of inept; a full caster who doesn't actually know how to cast, fight, or do much else, but at some point in their life, they attracted the attention of some very powerful being. Since then, the being has blessed them, often casting spells through them or imbuing them with some level of fighting prowess for a while. 

The problem is, I can't decide which class this should be a subclass for. Warlock seems closest, but it doesn't exactly fit in with the other subclasses. Any ideas? 

That just sounds like either warlock or wild magic sorcerer tbh.

I don't really see this as a subclass.  Subclasses have a mechanical difference between them the other options (access to different spells / abilities / features).  A lot of this sounds like less mechanical and more different in flavor, how you describe what your chanter does.  You could make this character a vanilla cleric, sorcerer, even paladin (might be funnier if he doesn't even know how to fight with the sword he is using).   You would just play the character like they have no idea how they do the things they do.  It's like there being controlled by a player, almost like it's a game.  I keep on picturing Jackie Chan when he appears panicked, and acts like he has no idea how he's still standing in the fight.