You were just chilling in your room and suddenly this freak ass weirdo intruded with a stray kitten in his arms. Spoiler: the kitten hated him and scratched his cheek.

or

You and Mello are friends/a couple/whatever you decide (I left it open for interpretation) and for some weird circumstances, you ended up living together. Now, after developing a strange obsession towards you, Mello wants to show you he is not all aggression and impulsiveness, so maybe that way he could steal your heart.

And what better way to do so than by adopting a stray kitten?

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