
Masato faked kissing Kou (Asahi's brother), causing the missunderstanding with the boyfriend at the time, he basically made everything worse.
Naru is a fairy god mother in disguise. It looks like he just wants Masato to stop running from his feelings, to deep in his heart change the view that relationships with hetero don't work - and that he doesn't deserve to be loved.
I seriously think that Naru wants to leave everything in the open, so they (Masato and Asahi) can move forward.
The thing is, Masato behaves like a coward, hurting people, spreading his evil intentions on every couple because his story with love is horrible (see the main story with the previous couple in Changed World). He needs to really repent. And yes, karma is a bitch.
Honestly, I'm a bit confused by the translation, how did he cause the first break up?