Let me clarify Gabe:

Gabriel is a boss. Although it has never been previously established just HOW old he is, suffice to say he's old enough to have met Cain in person, and have been worshipped by the Mesopotamians. He is absolutely ancient, and he was created for the purpose of being a mystical, mysterious, thing.

Now, the whole 'no magic' thing has already been shattered, this run has a lot more magic than previous ones (where Mith's pyrokinesis was a topic of debate, and Gabriel was the magicalest magic-man out there) so he's no longer as magical as he used to be. However, he's still massively psionic, capable of great mental feats. Gabriel can instil emotions in people, create illusions, generate insanity, read minds, and do many other things. He has never, however, 'mind controlled' someone. The act of mind control takes control away from a player, and this decreases people's fun. As such, I never plan on having Gabriel do anything like this. Instead, he merely shifted Kami's senses slightly, causing him to think the heart was a bit to the left.

To balance out how rampantly powerful Gabriel is, I added in several factors. First off, he doesn't pick sides. He has no intention of saving humans or vampires in particular, and doesn't care for either. He might interfere occasionally, for fun, but he isn't going to 'fix' the results of any battle. As such, he doesn't take agency away from the players - their actions are still what determines the outcome of the battle, and the story as a whole. Furthermore, while this has almost never come up, Gabriel is physically weak. While he's still stronger than a human by far, his strength is barely above that of a Newblood, and any Oldblood should be able to beat him down in a physical confrontation. I created this so that, if one takes into account Gabriel's capabilities, there is an effective counter to them and chance of defeating him. He's certainly powerful, but use of robots, drones or people like Mith (who are immune to psychic tampering) will nullify his greatest abilities.

Thus, to me, are the basic rules of making an overpowered character. If one 'side' has a truly OP character, then other character's actions become irrelevant compared to this overpowered character. In order to preserve agency, overpowered characters must always be neutral in a conflict. Furthermore, there must always be a method available to defeat them, if necessary they should serve as a 'boss battle' for other players to gang up against.