I'll call the couple Black and White, and the other one Pizza Sauce.
Black was childhood friend with White, who moved away, then after reuniting one day they eventually become couple.
Black became friend with Pizza Sauce during the time away, and while listening to Pizza Sauce talk about her similarly 'childhood friend, moved away, had a crush on her' she's like 'aww isn't that nice'
Then when Pizza Sauce said her childhood crush will soon be moving back here, Black asked to see a picture.
Surprise, it's the same person as Black's childhood crush.
So Black proceed to pizza sauce Pizza Sauce, and delete everything related to White on her phone to make sure nobody would contact her.
Then go do her childhood friend reunion.
tl;dr - Miki had 2 childhood friends who had crushes on her, the two met and became friends without knowing, and one of them murder the other upon figuring out then proceed to get Miki for herself.