In the final cutscene of Kenji's journey as serpent clan, who is the guy who climbs up and kills Nightvol with the pickaxe? Is that Grayback? Also what happens to him, does he fall and die too?

Thanks. It was a bit weird. Not really fitting. I suppose it was something that would have been expanded up in the sequel, but it was never made.

Yes, it was a bit weird that Grayback suddenly decided to help the Serpent Clan.

It was not sudden and he did so not because he wanted to help the Serpent, but because of his own motives that should've been understood by the player during the campaign.

For me it wasn't the fact that he'd "helped" the Serepent that was strange (I can totally see him try and kill Nightovl in the situation; as you said for his own motives), but rather the fact that he was put there by the designers in the first place. Last seen, he was killed in his base. OK, let's assume the wound wasn't mortal. I still don't see why the designers should decide to have him kill Nighthvol during his dying speech. It wasn't even a deus ex machina type of situation where Grayback appeared and saved the day. If he killed Nightvol with a pick-axe, Kenji could easily have done it too.
If this was meant to be the end of the story, then the addition of Grayback seems unnecessary. The only way I see this making some sense is if there was supposed to be a continuation/sequel that would expand on this.