Had an idea for Yveltal foe her milk tasting that she has eaten so much chocolate and never gotten tired of it that her milk is brown and also tastes liker dark chocolate and if anyone week was to taste it they would die. Thus death by chocolate or chocolate of death yet the MC is strong enough that he can enjoy it without worry.
Xerneas milk taste would change as the MC drinks it from different soups and drinks to fit with and ends with desert flavor since she has eaten so many diff types of food by then.