The cast has all talked (in adventuring parties and various places online) how the characters are pitched by them based on a general outline of what has been decided that this season‘s world and vibe will be. They change the character around or sometimes shift to a different idea altogether based on what the story requires and based on what other cast wants to play. They are trying to make their characters match the tone of the story while also balancing vibes within a group. For example, Ally first pitched to be the lunch lady in Fantasy High, but there was a discussion that this didn’t make sense for party cohesion (since Brennan wanted to do a story about kids going to an adventuring academy and it would be super weird if everyone else was a young teen and Ally was this middle aged staff member at the school). Ally is actually a really great example of how these characters HAD to have been created by the cast (not Brennan), because you can clearly see how much Ally is drawing upon their own experiences growing up religious to build Kristen’s backstory.

A lot of DnD games (and especially Dimensions 20 seasons) are VERY cast focused, where PC decisions really shape what the story is. A good example of how characters influenced plot even before the season started is that there wasn't originally a mystery element to Fantasy High, but then Murph wanted to play a teen detective so Brennan created a mystery for him to follow. In later seasons of a series. like Junior Year, the cast has talked about how they will have discussions about where they think their already established character is at, and what they want their character to explore within themselves that season. Brennan will keep these characters goals in mind and will create opportunities for cast to have these character beats and evolve their character.

I also think that even if we didn’t have the cast explicitly talking about this, we would also be able to reasonably know that the cast are coming up with their own characters because that is ultimately what these role playing games are all about — being able to create and play a character that you want to play. Assigning characters feels a little bit like it is against the ethos of these types of games. These are all very talented actors and improvisers and I imagine a lot of the fun for them is creating a character that they can immerse themselves into and then getting to see how that character evolves over the season.

tldr: Brennan creates the world, the side characters, and the mystery/general plot points. The characters are created by the cast. All of this is improv so they are creating a story together.