I got surprised to know that Serena was going to sync pair with Zygarde. I loved her outfit matching Zygarde's green colours. 
I was wondering if she would sync pair with Sylveon at some time, wearing a performance/contest outfit like in the anime.
But she already has Fennekin, Whimsicott, and now Zygarde.

Together they both look really OP!
Enjoy!