Leaf, Kris, Victor, and Elio/Selene are sort of worked in without a clear explanation. They have some protagonist experiences (most of the protagonists do regardless of whether it's their role), but they exist as a sort of "Oh yeah I was there and I'm a secondary protagonist and we're not going to go into too much detail about my mom or house". Starters don't really mean much at this point since a number of the protagonists and rivals are shown to have multiple, like May with Swampert and Blaziken, Rosa and Serperior and Dewott, Serena with Delphox and Greninja, Calem with Greninja and Fennekin, Gloria with Inteleon, Thwackey, and Cinderace, etc., though more often than not they have a "main" starter and the other starters may receive an explanation like Gloria receiving a Scorbunny egg from Hop.
Ethan is the protagonist and Lyra is the neighbor
Brendan is the protagonist/son of Norman and May is the neighbor/daughter of Professor Birch
Dawn is the protagonist and Lucas is the assistant of Professor Rowan
Hilbert is the protagonist and Hilda is the Battle Subway partner
Rosa is the protagonist and Nate is the Battle Subway partner
Serena is the protagonist and Calem is the Battle Subway partner
Gloria is the protagonist and caught Zacian while Hop caught Zamazenta
Rei is the protagonist who came from a different time and Akari is the girl from Hisui
Juliana is the protagonist in the Scarlet universe and Florian is the protagonist in the Violet universe
But we're left with Alola not having a primary protagonist, with both Elio and Selene being treated relatively equally but not having an explanation for it like Juliana and Florian.
