I read a story about a dude who accidentally summons a demon when he get pricked by some thorns on a tree, and gives him his soul to be good at magic.
The demon featured is name Mulok, which sounds like Moloch, but can’t be because 1. he doesn’t talk about eating babies 2. I don’t think you summon him for magical power.
But let’s pretend that you suck at magic. Like, your average grade is a G because F- was too high. And let’s assume that you are dumb enough to not consider the risks of summoning a demon because you are a dumbass YA book protagonist.
You have these options:
Asmodeus/ Asmoday: The only good thing he ever did was building Solomon’s temple, and that he had to be forces into. Teaches people how to turn invisible, answers all questions truthfully, and his favorite hobby is breaking up young couples. Either a fallen angel or the son of a human woman and an angel.
Astaroth: Very good at teaching wisdom and knowledge, magical or otherwise, but his breath is a noxious poison that you need a silver ring to protect you from and he will recite the story of the fall because he still thinks he has a shot at being an angel again.
Paimon: Loyal most to ol’ Lucy of all the demon kings, teaches occult knowledge and provideth good familiars. He is pretty neutral on humanity, though, and if you don’t hold up your part of the bargain, he will retaliate. I think he also wants to be an angel again.
Vine/Vinea: He is a black horseman that specializes in teaching the magical
arts. He can also make waters stormy and built towers. Also neutral feeling towards humans.
On Moloch, he appears to be a corrupted version of Ishtar’s consort, Adad, so you don’t have to find him so much as he will have to find you.
Then again, he could just be Tlaloc in disguise, he’s ugly enough to feature in the Ars Goetia. Jaguar Teeth, Buldging Frog-Like Eyes, Blue Skin, etc.
You can take this for further research: