
Alessandro is what his canon refers to as a Preternatural. He was born with the ability to neutralize supernatural abilities through maintained contact. Canonically, if he touches and maintains contact, he renders supernatural creatures such as vampires and werewolves to a fully human state. In short, he negates absolutely all supernatural abilities: genetic, magical, or otherwise and turns a supernatural creature into a baseline human (they would still maintain any skills or knowledge they possess. For example, if he grabs onto a skilled fighter, they are still a skilled fighter. They just don't have super strength or speed anymore.) For ghosts, touching their remains sends them off to the great beyond.
The theory behind the ability is that nearly all beings produce aether or soul. Those who produce a normal amount of aether are normal human beings. Those that have too much become werewolves, vampires, or ghosts. Those who don't produce any at all are preternatural. Basically, the amount of aether one produces is equal the amount of soul a person has. As someone who has no "soul", Alessandro basically removes the excess upon touch. It is an ability that requires direct physical contact that must stay connected. He can't just poke a supernatural creature and turn them into a person.
We don't know how this works on other beings (IE: Mutants/Super Heroes, Witches/Wizards with innate magic, Gods/Demons, Robots/Constructs), but we do know that being submerged in water inhibits the ability.
This is where the OPT IN/OPT OUT comes in. As a player, you are more than welcome to say that his ability doesn't work on your specific sort of vampire/werebeast/cthulhu monster/shounen protag/Alien. Or you can opt in and say that it does indeed work, or even if it only works for certain aspects
Below is the form
Player:
Character:
OPTING IN TO HUMANIZATION | OPTING OUT
Additional: if Opting in, what abilities can he nullify.