>>528598414
>Characters can have multiple arcs
Fair enough, but there's no canon distinction of where arcs begins/end. I'd say its subjective until we know how the writers are framing it.
>We also don't know that Riku is a deuteragonist
I definitely read the 3/MoM scenes as them keeping him a major character going forward, but I guess it could turn out like you said. another side was a similar situation and ended up with him pushed to background events.