@Raineh Daze@VitaVitaARFirst-person can actually improve differentiating between other characters within the narrative. Beyond just the physical description of other characters from the viewpoint of the 1st person, there's also how that character perceives the others.
For example, say the 1st person is talking to a pair of twins.
Now he might be classmates with one of them, and thus is more familiar with the dude. So he'd be more casual with the guy. But he'd be less casual with the other one due to their lack of familiarity, and an extra layer of awkwardness because the other twin looks familiar, but isn't familiar at all. There's also how the 1st person will perceive their differing personalities.
These descriptors can be woven into his viewpoint.
Or, if he dislikes somebody, the description utilised will be a lot more harsh.
(A brown-haired bloke breezed past me, his stick-like legs carrying him with excessive speed further into the distance. Douchebag.)
(The man passed by at an intense run, his beautiful, oak-coloured hair flowing against the wind. He was a fast guy, I noted. Real fast.)
They could easily refer to the same character, but the attitude of the 1st person towards them can differentiate them in the narrative. Even in crowded scenes, as long sufficient description is utilised, it wouldn't be confusing at all, especially with the whole new set of adjectives that being in 1st person provides.
To note the "main character" thing.
I wrote Matsuoka up with "supporting cast guy" in mind. No intent for MC-ing here, and it's functionally impossible for a character to become a main character in this situation anyway, hence there's no reason to try. I can understand your reservations perfectly, but rest assured that I'm pretty much using 1st person to explore Matsuoka's stream-of-consciousness inner character voice, which is a bit more difficult to pull off in 3rd, especially given Cloacina's presence.