Personally I feel like your story is flawed. It’s because your OC feels too... “overpowered”. Kamen Rider King makes no sense to me. He can somehow go to Helheim, create new Gaia Memories/Shift Cars/Rings, can summon a multitude of different objects, and yet he’s still a thirteen year old child. It makes no logical sense to me because creating Gaia Memories and Shift Cars are extremely complicated and is why only the Sonizaki family and Belt-san know how to make the two respectively. On the other hand the rings are mystical objects and only skilled at the craft can make them. I do like the concept of Kaijin that can posses others in order to become stronger, but your OC and his story needs more balancing and fleshing out.

I highly recommend that you make a fanfic about Kamen Rider King himself in order to get a better idea of what you’re trying to write about.

Even Decade is not that OP, and he killed every existing rider one time.

What I really like about this is that it stars actual riders and not OC's