To your point for *needing* new characters in roguelikes, I was just thinking of the gap you're trying to address of what I assume are runs not being unique enough. Ludologically a character in other games gives a feel of a significantly different run but requires more art, vs. if you only have one character, I think you still have per-run uniqueness in other ways, and if you want your player character to be more unique, you can build the personality of the character for the player to bond with them with story + quips and maybe through progressive upgrades. Just some thoughts around this, but I think you can be more flexible on your requirements.
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I agree, games can have a single character, but I've noticed a relation between how mechanically heavy a game and the breadth of the character roster. In this case I think new characters have a lot of benefits > more build variety, more to unlock, fan favorite play styles, different looks.
If it were a heavier story game and one where the character is the player I would probably keep it to one!