He's certainly adept at hogging the spotlight form more interesting plot points!
It doesn't help that it also took Zenos by surprise. He didn't know he could do it. You can pass off some of the powers he later uses to having shared his body with Elidibus (since Ascians can pick up knowledge this way), but that particular 'trick' wasn't one in his repertoire.
His in the sense that he had commissioned it, perhaps. After all Varis had expelled Aulus from the imperial court. With that said, I think Varis didn't want this potential nutjob causing problems in Garlemald but was willing to observe the results his rather insane son was able to get out of it. I am sure Zenos knows a bit about the whole topic, but the specific knowledge you'd need to hop bodies isn't something he claimed to ever have and it does take him by surprise too.
So he does not appear to have expected it and indeed, it's not something mere knowledge of souls will grant you. The Ascians have mastered this knowledge because they do understand the full spectrum of powers the Echo can grant. Zenos at most has a second-hand account from Aulus and the dialogue above does little to suggest he planned it.Katana-wielding Soldier: There upon the stage I stood, prepared to take my final bow, only to find that the finale was but an intermission...
Personally I am inclined to agree with Crushnight that he's terribly written, as well as on his other points. They've done little to bring out how things are happening. I can see elements in the existing lore that could support it - for example, his sword serving as a vessel for his soul (albeit even this requires a stretch), or picking up soul scrying knowledge from Elidibus, but they do precious little to explain this, so it really does come off as magic mcguffins. I am not convinced they intended to keep him around until they saw his popularity, which even by mid SHB had waned quite a lot, according to a poll they did in Japan on FF character popularity.
Anyway, much as I now find the character rather insipid, I do wonder if they'll tie Zenos to Azem's soul. There’s some issues with the idea, like the fact that all of Azem’s surplus shards would be Reflection ones, but keeping in mind who his great-grandfather is, and what he could do, maybe there’s some merit in the idea.
A lot of the time I see Hythlodaeus posited as his soul shard, mainly because of the “my friend, my enemy” thing, however I don’t really see any strong resemblances beyond his references to the MC as a friend - Hythlodaeus, along with Emet-Selch and Elidibus actually had a friendship with Azem, unlike Zenos's one-sided friendship with the MC. This post makes an excellent point in reminding us that Zenos’s Resonance is based off the template of Krile’s echo, so if anything, it’s likelier that she links to Hythlodaeus than he does – assuming either of them do. Granted the post has some inaccuracies but I think the point about the Resonance is decisive.
With that out the way, Ardbert makes some interesting comments about the loneliness he felt when he was a soul drifting about and how it makes the Ascian grief more understandable to him, even if he thinks it’s a bad mental place to be. A difference with the unsundered Ascians (and even the sundered ones whose memories are restored) is that they at least could connect to their own people, even if they can’t relate to the sundered life forms due to the gulf between what they knew as their people and what they see as pale reflections usurping them. So their aspirations are directed at restoring the world to what it once was in its complete form.
In Zenos, he seems to also share this inability to relate to others around him, but without the context that the Ascians have of their ancient lives to form a goal. He has no notion of this. With Ardbert’s (a shard of the MC) comments in mind, it does seem like a rather similar sense of angst/ennui, which in Zenos’s case can only be remedied through sating his bloodlust. Relevant parts quoted from here and searching for 'Ardbert' in case anyone wants to read the rest:
Ardbert : J'ai beaucoup souffert, que ce soit du déluge de Lumière ou de voir la réputation de mes camarades salie. Mais en vérité, je portais en moi une douleur bien plus
grande encore. Et cette douleur avait un nom : la solitude.
= “I suffered a lot, whether it was the Flood of Light or seeing the reputation of my comrades soiled. But in truth, I was carrying an even greater pain in myself. And this pain had a name: loneliness.”
Ardbert : Plus encore que le temps, la solitude est assassine. Il se peut qu'Emet-Selch et les autres Asciens soient rongés par cette même sensation de vide…
= “Even more than time, loneliness is murderous. Emet-Selch and the other Ascians may be consumed by the same feelings of emptiness…”
As to why Emet may have put the soul shards in him, perhaps it was his last ditch attempt to see if he could yield something he’d recognise as worthwhile out of the sundered life forms. Having witnessed his “son” die in spite of the high hopes he had, perhaps he thought “if I can supply a strong enough soul to the body, perhaps that will do the trick”. All the same, Emet does not once remark upon him, and Elidibus says nothing of any resemblance between the souls of the WoL and Zenos, other than to call them both “beasts” – he may have forgotten who Azem is but it’s not required to recognise a similarity in hues. Zenos witnesses visions of the Final Days but in a clearer way than the MC or anyone else we’re aware of does, and there’s also no indication he even hears Hydaelyn’s call – or if he does, has any inclination to act upon it.
That’s one thing I am wondering if we’ll see change, since there are still so many mysteries surrounding Hydaelyn. Especially with the possibility that Fandaniel may well not be the original seat-holder – given what we know of Elidibus departing Zodiark, it is entirely possible Venat (whose name means "to hunt") did the same at some point and if so, he could be a resulting sundered shard of hers, again completing the reference to Venat guiding Vayne in XII. Sort of. Then you could get Anima out of all that, as some twisted representation of Zenos’s “mother” (although I still think there’s a strong chance it could just be Zodiark’s residual consciousness following his sundering.) Maybe they'll even have her shard be who Zenos is, because there is a point in her story that Azem ignored their overtures, but you could also have that with a Zenos-Azem and Fandaniel-Venat storyline. She finally got her man, so to speak. Assuming either/both of the duo are ancient souls at all, and not something else.
It’s not really definitive, but with all the talk of Zenos being a “mirror” to some players and a foil to the MC, it’s one way they could write something of a sob story for him without him necessarily being sympathetic – more so a tragic tale of how the shards of Azem could wind up. He may be some other soul or something other than an ancient, but the foregoing is at least plausible to me.
Of course with Yoshi pointing out that Zenos thinks of you as his “friend”, I suspect they’re going in the Golbez direction with him. I just hope by the end of 6.0, he’s gone. IMO we’ll probably see a fusion of IV and IX primarily, so I expect we’ll see a fusion of some Necron and Zeromus themes in Fandaniel in some respect. The way Zodiark is guarding the “moon” in the Amano art makes me wonder if it’s 1) perhaps actually the Source or 2) some manner of ark. I’m very keen to see how they proceed with things, Zenos aside.