Do someone can help me to have some ideas?

I have 4 characters and no idea of wat to write...
I have my protagonist, a human magic warrior from midgard, who escaped from Valhalla.
An cupid who lost his memories when he hitted his head on a tree and entered in a pregnant woman and born as his child.
A demon who pursuits the cupid but lost his memories and apeared as a human.
And finaly, a normal female humanbeing.

I need a idea that involve all this worlds, and a common enemy of all worlds