Perpetua, outside of her boss fight, has not sold me on being anything but a joke character.
She may mostly be played for comedy, but in terms of being a candidate for endgame plot relevance Perpetua has a huge advantage over basically every other character.
Because of how the game is structured, getting to know any of its characters is all optional. Some of them you have to fight once to progress, but afterwards you can completely ignore them. Which is a problem if you’re trying to pick a reoccurring character to play an important role in the final dungeon.
Like imagine if Venefica were chosen to do so. She’s powerful, serious, is considered important in universe… but she’s also siloed away in her own missable section of the game. If she showed up to do something grand near the finale, a number of players would just be left wondering who the heck she is.
Perpetua is the exception to this rule. She shows up multiple times on the critical path. You have to fight her at least three of those times. Repetition alone makes her memorable. And while some of her characterisation is over in the optional slime village, I think she might have more unavoidable dialogue than anyone else.
So purely for the meta reason of ‘every player is guaranteed to have had lots of interactions with her,’ she’s going to be relevant.
((Plus, you know, she wants to replace the Demon Queen and is a colossal fan girl of the Queen’s second in command. Not seeing her interact with either of those characters would be a missed opportunity.))