In Dweller there's a Bag of Monsters that you can come across while exploring the dungeon. When opened a charmed monster will pop up and fight along side you. It's the same as using a Book of Charms on a monsters, but it gives the warrior and ranger a chance to get an ally. Yesterday I added a Dog whistle and gave that to the ranger as a part of the starting gear. The whistle has an unlimited number of uses and when used it summons a dog (although you can only have one dog summoned at a time). The dog will work as any ally and it will try to attack targets adjacent to the player.
I am now contemplating if I should give the wizard the ability to summon elementals, but I'm afraid of the difficulty balancing this. The wizard is already powerful as it is.