Booster is Pixel's self-insert

Booster has journals on the island, given the one in the Prefab House. Upon his death he states "I pray for your victory." The waterway room is internally called "Pixel" (& there's the "Wife bore child" message) and the person on the computer also records in journals and states "I pray for your victory" after Curly is drained.
Booster additionally talks about "pressing the Jump button" and "the Item Screen", being (I believe) the only non-Cthulhu character to mention fourth-wall elements.

 

Sometimes we have kind of broad definitions for what qualifies as a "self-insert", but yeah, I guess in the case of Cave Story, Booster probably is the closest thing there is to a self-insert from Pixel. People tend to do that kind of thing with their "wise" character. Like J. K. Rowling did with Dumbledore, and like J. R. R. Tolkein did with Gandalf. So Booster kind of fulfills similar characteristics.

 

You'd think it was Kazuma, considering the how similar he looks to Pixel.