I don't mind the new guy. He looks like he's gonna be the type who's just genuinely stupid and will support the main couple from the back lines like the other girl.
The thing for me is he's not an introvert like the other girl is - being on the back lines doesn't fit his character shown here.
And, even if he's not a true rival, he needs to appear as one or else Mizuno won't be pushed to process his own feelings toward Wakao, so he needs to be actively involved and his actions and words need to be just ambiguous enough to keep Mizuno off-balance. And that's only if he's not a true rival.
Because now that he
has been introduced, he's a part of Mizuno's romance arc, if he's not getting in the mix as an obstacle for Mizuno's connection to Wakao, then his being this extroverted serves no purpose.
He might ultimately not be a rival for the ML, but he'll necessarily be actively causing Mizuno pain for the duration from a romance angle, I think.