Mami is easily the worst character of the manga because she was made to be antagonistic, she is wishy washy, dated mc before but instead of telling him what's wrong with him she just dumped him but then takes him for granted whenever he is around, bad-mouths him knowing he still has lingering feelings for her and capitalises on them and can't stand a clearly better girl is attracted to him and that MC also has feels for said girl.
Mami simply doesn't deserve anything because she expects everything to fall on her lap. Even this MC as flawed as he is doesn't deserves such punishment.
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Now if you read this enjoy the usual autistic comments hating on the MC.