I would argue that Mia is the lead protagonist as it’s her story for the first nine pages and arguably the end vision is really about what her life could have been — we see that Sebastian owns his club, but we know nothing else about how his life has ended up.
If Mia is the protagonist, then Sebastian is the attractor as the story is about how this love helps her to believe in herself, which leads her to achieving her professional dreams.
The most interesting thing to me about La La Land is what the movie is ultimately trying to say about love and success (and I haven’t figured it out yet). I’ve seen the film described multiple places as a “love letter to L.A.” but I walked out of the theater feeling that the main message was that you can’t have both. It’s either love or career.
Mia’s first boyfriend, Greg, seems like a Bellamy — he’s nice enough but he’s too superficial for her, talking as he does with his brother and his brother’s fiancee about home theater systems. While her husband, David, isn’t really described in the screenplay, in the film he seems like Greg 2.0. That would mean that Mia ultimately settled — she got the career, but she ended up with a nice guy who isn’t quite right for her.
Sebastian, on the other hand, tries to make the opposite choice — while I think Keith serves as a trickster for him, testing to see how dedicated he is to his dream of owning a jazz club and playing real music, Sebastian doesn’t actually choose Keith’s gig for the money. I would argue that he chooses it out of love for Mia — Sebastian tells her he doesn’t want the gig, but he changes his mind immediately after hearing Mia talk to her mom about him (p. 45). I feels like he does it to prove himself worthy of her. If you buy that, then when Sebastian chooses love, it leads him away from his dreams; when Mia chooses to settle for the nice-but-boring guy, she achieves hers. So, in fact, you can’t have both. My sense at the end is that their lives are emptier for the loss of each other.
The nemesis character switches around, but it’s everyone who keeps Mia from following her dreams — her coffee house manager, the casting director that interrupts her, the director who immediately cuts her off, even her mother who doesn’t seem to really support her. Sebastian has the same sort of foes in his life: his never-seen business partner who swindles him out of the Van Beek, Bill, and the singer of the ’80s band.
Mia’s roommates disappear early in the movie, but they function when present as sidekicks or allies. It seems like Tracy might even be Mia’s manager or agent. Amy Brandt seems to me to serve as Mia’s trickster, making her choose between a wonderful career opportunity and staying in L.A. with Sebastian.