You REALLY need to look into the lore of Melty Blood to understand why there are so many variants of characters..it's not just a fighting game but it's also a Visual Novel in the Type MOON universe..kinda like the Fate series. They are both put out by the same people and both get kinda nutty with how many variants there are to the stories due to originally being Visual Novels. Type MOON has some messed up stuff in their lore and the various endings of the novel. Shiki has what's called "Mystic Eyes of Death Perception"..he literally sees lines everywhere without those glasses of his and it started to slowly drive him insane because those lines are linked to destruction..if he breaks those lines with something like a knife, whatever he did that to is shredded or cut to bits. Seeing the literal lines of Death that make up the whole world and people was driving him crazy cause no one else saw them. Hit up the Type MOON wiki to learn more about the characters and story.