I do kinda wonder if they were friends before they developed feelings for kei or if it happened concurrently.
In my head it’s kinda like a love tearing friendship apart thing where they grew up and both had feelings. The male ml realized it’s more societally correct for kei to be with the female ml so he pushed her away from kei in middle school, and now we’re here.
Ofc we don’t know the specifics, they barely even talk to each other just give death glares before pulling kei along like a doll.
It really reads to me like they both like Kei, and the general dynamic of how "gender cliques" have changed & played out at school since they were kids, have pitted them against each other.
When they were younger, it was more "acceptable" for Noel to hang out with Kei. But as they got older and into middle school, "boys couldn't hang out with girls", and so Kei ended up with Eito more, and Noel was pushed away. Coupled with how meek she is, Noel couldn't really do anything about it, and so she felt like Kei was "taken away" from her.
Now that Kei's swapped from masc to femme, Noel's got an "in" with her again, and can be close to Kei without it being socially weird. Which has put Eito on the outs -- except for the fact that
Kei still operates like a "boy" to the point that the continued hanging around with Eito is throwing Eito's feelings out of whack, and causing a lot of this tension and discord.
And because Kei is convinced that Eito & Noel have feelings for one another (at least to some extent), Kei is oblivious to the truth about how Eito and Noel both have feelings for Kei. And because
they won't confess that to Kei, we have the ongoing misunderstanding.
That's my read on it, anyway.