Not getting ahead of the story are we? ;)

Ummm. I really don't know... The sorceress shopkeeper in a weird anthro urban fantasy game. Sells you cookies that do wierd things.

Alright, I'll do one where I pretend I don't know who you are already :p

A powerful swordsman who uses a giant sword one-handed. Decimates the part on their first encounter but joins them later for unknown reasons.

Who ARE you cosplaying?

And...the newbie trainer/tutorial person.

Espy, go to the writer's forum and look at the story Quiet posted.

A boss that misses a lot of hits due to his blindfold, but fatally wounds anyone he hits.

My gut instinct was right, then...even though I haven't played most of the FF series.

A little girl who is turned into a cat by a magician. (Hm...I'm partly playing along with the FF theme.)

The Final Fantasy part is only marginally relevant. Purely esthetic, no plot relevance...for the most part. ;)

Competing spy assassin that keeps getting to the mark ahead of the party for story purposes and it really annoys the players because there's nothing they can do about it!

a lost hero, who later finds their way

Powerfull monster. :) The kind the party usually prefers to run away from.

an unlockable 'created' char in a melee style battle game

Mean winter weird encounterd in snowy mountains.

The protagonist's sidekick who is reasonable and level-headed, unlike the protagonist himself...

That would be the other guy ;).

The blind witty leader of the assassin's/thieve's guild. Either one.

The hero who's also the comic releif, and only shows his serious side in battle.

The hero's little sister who is shrunken and battles rats and snakes with a pen she found.

The spunky white mage that sometimes does more harm than help~

The Warrior character because No Berserkers are allowed.