When you get the Slime early on it is kinda cruddy (but still useful for the Darry+Barry fight), but as long as you've been feeding it, once you hit mid/late game it's stronger than Aura (excepting the fact that it has a Darkness weakness). It's Wind form is also practically a prerequisite for underwater combat. The Slime is also viable for the Alchemist when you get the Morph (Fire) skill at Slime Lvl 5. Note that the slime has to be unsummoned when you talk to the girl who taught you the summon spell, the game is not great about telling you this - I honestly got the Slime to lvl 7 before I figured that out on my first run because I practically always had the thing summoned.
One caveat with it being stronger though is that the Slime is definitely a lot better with the Bonding 1-3 passive (involves lewd scenes, will drop Victory Score).