[Kiwami]

The crucial difference could be that Kaito was hurt by an Overlord Inves, not a regular one. The Overlords were genetically altered by their king to adapt to the forest. Maybe Kaito somehow got that.

Haven’t we had someone claiming Baron and Gaim fight in the final episode, showing pics? Baron was in normal Arms there, not an Overlord.

Maybe Kaito becomes an Overlord, although it is what he wished for. Seeing as how he values strength and evolution to a higher state, I find it fitting for him to become an Overlord.

In the end he could be restored when Kouta "heals" the world and both fight a fair and square battle in the last episode. I could even see a "you don’t know who wins"-ending happening, when we see Kouta meeting Kaito, both talk and transform and as they storm against each other the series ends.

I know a lot of people who would hate this kind of ending, but I wouldn’t mind.