Why’s Henri’s Soul Different From The “Norm”?

Henri were forced to be born prematurely since Annabell (Henri's mother, shapeshifter) were dying to a disease that eats your magic, since without magic, monsters die, killing her and the baby slowly. Richard (Henri's Father, Gaster) wanted to save them both, thinking if he could capture Henri's soul into an incubator machine he had built few years back to help the monster population stay decent, Annabell might get enough energy back to survive the disease to the point where a cure could be found.

She died shortly after the birthing, Henri barely surviving in the tank himself, disease eating his tiny soul up.

A monster, The Seer, with a sight of Past, Present and Future comes to Richard and offers to give his soul to the baby, so they'd have enough strength to survive the disease. "It's either this or killed in an accident tomorrow morning. Best not waste a good soul. And I've seen him to be a key to victory in the long run."

Reluctantly, Richard accepts the Seer's offer. There's a difficult soul transferring operation, but in the end, Henri's state is no longer critical, but The Seer has died and turned to dust. Underground had lost 2 valuable monsters to save 1.

Richard let Henri grew up more in the incubator until his new soul had stabilized enough to let him grow up like a normal child, having to leave the stitches on the soul to keep it in tact. His skeleton powers were mixing with the Seer's soul, but no one realized it, not to mention, due to the mixture, his mother's shapeshifting became the main source of his powers for being most stable. He could still use normal skeleton powers, but it'd be a 'one shot - lights out' (Bones&Blasters) kind of magic, he wouldn't be able to control it while his “Shortcuts” (Teleportation) mutated into “Time & Space Walking”. He can still teleport, sure, but he’ll be lights out after it.