Chaos

'''This is not FF7 canon. Well, some of it is. Most of it is not. This is mostly me trying to keep my notes straight. Spoilers abound.'''

Lord of Entropy, squire to Omega, he is the god of death and decay. He and Hades are sometimes confused with each other, but they are not the same thing.

Chaos and Jenova had A Thing going for a while. When he realized she just wanted to devour every soul on earth and basically kill his world and everything in it, that was kind of a deal-breaker. It was an ugly breakup. Chaos was blamed for a lot of things that were actually Jenova’s fault, which is perhaps why so little evidence remains of his cult.

When Lucrecia implanted Chaos' summon materia in Vincent’s heart, their spirits became bonded. In this sense, as long as one of them is alive, the other will survive as well. Likewise, you’d have to kill them both if you wanted one of them dead, and that is freaking hard to do. Although he is the strongest of the limit breaks and has been here the longest, he is always the last to trigger, primarily because he is so powerful.