I would also say she's a swordtail, swordtails and platys can crossbreed with each other so if you don't want fry or hybrids for that matter it'll be worth keeping them separate- none the less though if you are finding females for male swordtails and platys try to give them females of their own species as more than often they pay no interest in each other sexualy and it won't properly balance out the genders in each species.