A-G[]
The Cynic: An oddly "genki" version. Isshō, at the least, views humans as worthless creatures who only squander what the world has to give them. However, he rejects the ultimate conclusion that nothing matters; if the humans won't enjoy what they have, he'll live his life different from theirs.
Darker and Edgier: At first, Isshō was created just to be Darkrai's obligatory trap character, and mainly used as an excuse to get some easy access to Lightning-Fire Dragon Slayer, back when he was very bad at what he does. As of Isshō's revamping, Darkrai has abandoned the Slayer Magic, given Isshō a less fanservicey (but still very feminine) appearance, expanded on his backstory to include a very nihilistic view of humans, and made him so obsessed with staying alive that he willed himself back to life.
Eldritch Abomination/Humanoid Abomination: Both Darkrai and Isshō himself make a point to differentiate the boy from both humans and demons, and which kind of abomination he is depends on how you view him. As Isshō was created from the very essence of the world (magical energy, specifically Eternano), he could easily just be seen as a Humanoid Abomination, the world in human form. But Isshō specifically mentions the magical energy that created him was powerful remnants of past wars where massive bloodshed occured, which would place him firmly in Eldritch territory. And this isn't even taking into account that in the Sun Trilogy, where Isshō exists, the entire world is blanketed by a limitless river of magical energy referred to as the "Source of Magic", which could easily have had a hand in bringing Isshō back from the dead.
H-P[]
Healing Factor: So long as there is magical energy in the air, Isshō can heal indefinitely.
Immortality: A recurring theme in several of Darkrai's characters, Isshō has a combinations of Type II and Type III, Biological Immortality, complete with The Ageless, combined with a very powerful Healing Factor (see above).