https://awoiaf.westeros.org/index.php/Vaella_Targaryen_(daughter_of_Daeron)

Yes, far as I'm aware she was only briefly mentioned during the Great Council that appointed Aegon V where they decided not to crown her. Described as 'simple' what that means exactly I'd leave up to the applicant to interpret.

Maelys does sound juicy doesn't he? though I'll admit he needs to be handled carefully and you might not have [i]quite[/i] as much freedom with him as other claiments, what with him being one of the most plot important characters. I'd like their to be a good buildup to his actual invasion in order for everyone to be forced into shakey alliances etc