So for Yveltal her personality could be that she does not like the way she is. She has no real intent to cause destruction or kill its just the powers she was born with that don't really leave another way to exist without causing destruction. She may be a dark type but it does not mean she likes what she does to be a counter balance to Xerneas. She is jealous of Xerneas for being loved by many and she can only be feared or seen as a weapon and attacks xerneas. She could secretly wish to find someone that can love her for who she is and not what her powers do and to feel life being made inside her instead of taking life from around her. The MC cum can have the energy of life so she would not need draw life energy around her and can keep her from forming the cocoon or at least shorten it to need a few hours and comes out again. This disruption would lead to Zygarde going to the outside of the MC ship to face him. This can show that not all legends like the powers they were born with and don't really need to have a personality that matches their type. She can treat what she does as a job she does not want but as something to prevent worse from happening and will do the job. Plus she can do a lot more destruction if she actually wanted to but does not unless pushed towards it.
Also I can see Diancie being a beauty queen personality and having the MC to have a Dimond tip dick just to get into her.