Sounds like a pretty crappy all around player...loose/passive which is terrible. If the problem with them not raising is your inability to place them on a hand you're just going to have to treat them like you do a tight passive player and assume that they have a good hand. Tighten up and play them aggresive. Bet your big hands very strong and try ensure that at showdown he/she only sees you show down big hands. You also want to take into consideration that whomever this is, is chasing draws and calling down bets to the river with bottom or second pair as well as playing Ace-rag and K-K the same. You absolutely can't bluff someone who won't fold and this person sounds like that kind of player. Never raising or folding, constantly calling bets and chasing draws to the river. This kind of play can be extremely annoying, but is fairly easy to combat by simply playing good solid TAG poker and not letting this persons' strategy knock you off of your game.
You're basically going to be playing "showdown" poker with this individual and as such have to be able to make sure you have the best hand when you do go to the river with them. When they're chasing gutshots and backdoor flushes you have to make sure you have the best hand at the river and make sure his draw didn't make it. The upside of playing this type of player is that they will call with anything, so when you do have pocket Aces or Kings and hit your set on the river, you can extract tons of chips from them. Same scenario with flopping the nut straight or flush, you can bet the hand with each street and extract a ton of chips from this player.