I have no idea where Mokou and Marisa being friends comes from, but I like it and would like to know more.
The artist's favorite characters seem to be Marisa, Sakuya, and Mokou, and they (possibly she) like to ship the three interchangeably. I don't think it's anything deeper than that.
Completely slipped my mind. In AoCF, the Marisa/Mokou duo has several Touch-of-Death combos, which means if you get hit once from full HP, they can potentially combo you to death without you being able to stop it. That's why there was a sudden influx of Mokou-Marisa content from several artists.
