I only have the first prime game but I don't remember a thing about Samus. Then again not I like care about characterization in Nintendo games. It's just there to facilitate why you're doing stuff in the game. The original one's premise was pretty much "You are a bounty hunter in SPACE and there's some aliens that need to be shot at that have a super dangerous alien that needs to be shot. Here have this big dungeon to crawl through." The reveal was nifty though.