There are a few ways for that. You can also hire a merchant, than your group has already 3 members. Also some quest NPC are immortal, they can also be useful, but only for special areas like in sewer of horrors. If you use them wisely they can be a great help. Mages can also have golems (clerics can have wolfs) and all classes can summon a bear, but only for a short time.
That's kind of all possibilities you have in the game.
Ps: Don't talk about mods here, it's forbidden