Lwa are spirits of vodoun, and rather than remaining in their "in between" of sorts, they interact with their people. This is hazardous though. Damballah himself is the serpent god, father figure, lightning and ancient, unknowable wisdom. Unknowable in that he doesn't communicate it very well at all. Were he to take direct control of Guy, it would overload him entirely, burn him up. It'd kill him. It's a similar case for Ogoun, though less catastrophic. More information on both of them is here. He was offered up to be a vessel by his father before he was born. Similar case for his older brother, and for Papa himself. It's not something he can really help, per se. But he has a degree of will over whether either of them manifest.

Manifesting Damballah is a bad thing. It means that some shit is going down which Damballah believes only an ancient, powerful energy could stop/hold at bay. It wouldn't hold up for more than a few seconds though. Manifesting Ogoun gives Guy a sort of Berserker ability, but at the risk of burning him up if it remains for too long. It drops his defenses outwardly, sends various systems (respiratory, circulatory, etc) into overdrive. Think like forcibly running something your computer can't handle until shit catches fire.

So it's something better off avoided.