Windstar
[12] Conqueror
Arngrim belongs in S tier. He is the only non Lenneth character that needs to be in your party for the whole game. Lucian you get late and have to give up almost immediately to get the best ending. Brahams Freya and Lezard are only available during Seraphic Gate. The tier list should reflect characters you can actually use during the bulk of the game. Only 2 characters are S tier: Lenneth and Arngrim.
Lenneth is only S tier when she's carrying a bow. She pretty much outclasses every other archer in the game. Arngrim is certainly S if only because of his PWS (or Soul Crush). Mystina is pretty much a step above most mages. While she doesn't quite hold a candle to Lezard she's the best mage in the main game, unless in hard mode (and she's like Arngrim and stuck with you for the rest of the game).
Lawfer is the better of the two lancers. Janus is good, but not great. Every other character ends up being decent to being awful in most cases, though.
If you're looking for a decent party, though. Jalanda (until Mystina), Arngrim and Lawfer will satisfy all your needs. Just use the exp orb when you need to send someone up to Asgard.
Star Ocean 3 Tiers
A
Cliff/Mirage
Peppita
Nel
B
Sophia
Albel
Maria
C
Fayt
Roger