Xerosic is another on the list of XY's bizarre characterization choices, as in the main story he only appears once in the basement of Lysandre labs, giving a relatively weak fight (two pokemon? Slow down buddy, don't over exert us). Still they make this encounter very memorable as this leads to the "oops I made you activate a nuke" button choice. Which is certainly something I remember (no accounting for quality or taste though).
But beyond that he's a rather late-game addition that could have gone to another character, and yet he's the one that gets fleshed out in the XY post-game sidequests. But if you don't do the post-game, then I can see how you'd forget about him.
Doesn't help that UltraSun & UltraMoon almost retcons his biggest moment over to Lysandre (again the "oops you activated the weapon" two button trap). In XY Lysandre was against Xerosic pulling such a stunt, but in UltraSun UltraMoon Lysandre makes it sound like he was the two button trap guy all along.
Meanwhile AZ has the most significance to the plot but only appear four times total, and yes that's more or less equal with Xerosic but one of these guys is just a team Flare scientist and the other is a 9ft tall nuke-making immortal king of Kalos! Why does Xerosic have more a presence than AZ?!
I swear one of these days we will get the interview explaining what the heck happened with Gen 6 (like AZ's Floette, etc). I mean, I think I know the answer (assets cut off to make Gen 7 an anniversary celebration) but I still want to hear them say it.