User blog comment:Miskos3/Fairy Tail 522 & 523: Gray's Trump Card & Is Destiny Burning, Review/@comment-4933386-20170220013827/@comment-7418318-20170222212737

Eh? Why bring up Kain? What does he have to do with anything?

Obra was (or is?) Zeref's creation and servant so it would make sense for him appear in an arc where Zeref leads an army. And his ability to drain and nullify magic would certainly be useful in a fight against mages.

And Kain? He has a very tenuous connection with Zeref at best. GH wasn't really trying to serve Zeref, they just thought that their goals temporarily concided. Or you could even say they wanted to use him for their own ends and didn't particularly care what Zeref might want. Hardly what you can call true followers/subordintes/servants or even allies to Zeref. Just look at how they were willing (and actually did) beat Zeref unconscious to drag him to their airship against his will.