So I am working on a story where the intention is to have a mare fall in love with a parasitic being, which has the ability to mind control its hosts.

Since she is the first host he allows her to have control of her body, so long as she doesn't do anything that would result in him being discovered or endangered.

Since I want this relationship (the romantic aspect) to be consential, what sort of actions could the parasite do to gain the mares trust and eventually steal her heart? Any thoughts or suggestions?

6579037 Tough one. The best I can do is suggest that the mare in question is chronically lonely and desperate for attention, such that she'll even accept validation from a brainwarping parasite.

6580242 Is there a particular background pony you think might fit the role?

6579037