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.
http://tvtropes.org/pmwiki/pmwiki.ph...EquippableAlly

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.