I'm guessing that they're called parasites because they actually have Klaxosaur (related) bood in them. The difference lies in how much one could take, with double and single digits having more amounts of parasites in them.
If I'm correct, then I'm also going to guess that Oni actually slowly turns her partners into her, or something similar to her. I think that Hiro's body is undergoing change and that after riding for the third time (the mission will most likely fail) he'll collapse and be hospitalized for one episode until the change is complete (and his body has regulated itself).
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