Also, other "monsters" (with various other abilities and transformations) are trying to kill them, in an attempt to collect enough alien coins to unlock the power to destroy the world. (Not a word of this is an exaggeration.)
The season starts out strong, but I felt like it very quickly fell back on fairly predictable shonen tropes, without spending any time on the much more interesting psychological implications of the main characters' unique bond. They're very different people – a scared, reclusive nerd and a borderline-sociopathic gyaru – and the way they complement each other's issues and weaknesses, both in and out of the costume, had a lot of potential for interesting character work that was ultimately squandered.
But then, given the parade of obvious shonen villains in the OP, I was probably expecting too much from the get-go.