I think maybe that it'll be better if she can masturbate all she want but can't cum by herself. That would make Yveltal into someone who always crave to cum, in a perpetual heat and it would not be helped by Xerneas who would always tease her without touching her in any way but will make herself cum in front of Yveltal. They both love each others and even if she can't cum Yveltal love this teasings from Xerneas who's a real slut. Xerneas, who can cum when she want, would be submissive and sweet in her sexual encounters. She already got fucked by people who reached her but never got pregnant because they don't have the MC metamorph's cock. She would act all teasy and submissive. Yveltal, on the contrary, don't have many encounters, at least way less than Xerneas and is obsessed with the idea of getting fucked again and again. That make it so that she can't cum for years, years filled by Xerneas' teasings. The encouters she have are when people found her by accident while searching for Xerneas and she's a godess of death because she fuck them again and again and again until they can't take anymore, and so Yveltal would be dominant.
What do you think of this ? ^^