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My headcanon is just that Cross chose Nightmare over Dream and met Killer. Killer tries to be his friend after seeing how cool Cross really is (he was jealous at first since Cross rose high up the ranks after just joining) and after a while, he gets saved by Cross during an attack from Swap and falls in love <3.
I picture Killer's confession like they're on a rooftop of some au and fireworks set off and he just confesses and well Cross accepts like shshjfuddvsh 😭‼️