Equippable Ally Romances? (tvtropes warning)
Caution the following tvtropes link may lead to spending a huge amount of time exploring all the tropes if you aren't already familiar with the site.
To summarize, this is a fantasy or science fiction trope where one character can transform to be used by another character. The transforming character may become a weapon, a mount such as a dragon or warhorse, or other object which is used in direct partnership with a person (a 1-man spaceship/airplane/boat, a mecha, a suit of armor...) This just sounded to me like it could make a really great setting for a romance. And I wondered if any romances like this existed yet, that you all knew of.
There are a shit ton of dragon romances out there where, usually the guys, transform into a dragon that the leading lady can use to whisk herself away to safety. One that comes to mind off the top of my head is Master of Dragons from Angela Knight's 'Mageverse' series.