The real Self Insert is Plankton.Plankton and the author are both named James. (Plankton's second name is James.) Both have height issues, and C'ren is inspired by his high school crush who turned him down for being too short. Also a possible reason why he, of all characters, is the Only Sane Man in the story. Aside from that, both James and Plankton have dark hair and pale skin.
The real Self Insert is Fred Rechid.He's the Only Sane Man in the story, and by the point in the story where he wrote the newspaper article against C'ren, it is likely that James's true personality is already starting to take over.