All the characters have animal tail an ears apart from those with animal quirks (hawks ect.)

Izuku : bunny
Bakugo : wolf
Shoto+Dabi : cat
Kirishima : dog
(these are just examples)

A is an omega and theyre starting to give up hope on a mate and get really down when their friend B grows distant.

A becomes depressed and is about to give up on everything when B confesses to them that they always loved them.
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Alternatively B is a stranger that stops them