Morte - Three words: Talking, flying skull. Nothing more really needs to be said other than that. But, for seriousness, Morte is probably one of the best passive characters I've seen for a game. That said, Morte is the cynical, sarcastic companion to the before-mentioned
Nameless One. Morte offers comedic relief with his crass wit and glib manner, while also being a cornerstone for a lot of the character developement of the
Nameless One. Other than character developement, he helps the player a lot through-out the game with his knowledge of the Multi-verse and other tid-bits of knowledge and know-how, as well as being a resilient, and.... somewhat.... steadfast fighter.
Plus, he digs dead chicks. I mean, c'mon, is that an awesome character quirk, or isn't it?
Really, just about any character from Planescape: Torment can be listed. All are pretty memorable in their dimensions. I listed Morte because he really fits my ideal of a companion character, in practical applications or otherwise. (Maybe that's why I don't like Alyx from Half-Life 2 so much?)