Kishuna gets very little characterization by virtue of being completely mute. On the other hand, I am extremely unwell and find this to be a very attractive trait, and have devoted my tormented yet beautiful mind to the noble pursuit of filling in every blank left behind by the vile and neglectful intelligent systems writing team.

While he shows up in a variety of situations and usually disappears without seeming to have done anything, I like to think that he is deliberately leading the party towards eliminating morphs and foiling Nergal's plans. This isn't out of resentment towards his creator for casting him aside (if anything, he appreciates being given the chance to experience life), but rather because he fundamentally objects to the use of living things as weapons and can't see any future for his siblings but to be forced to fight to the last breath. All he wants is for them to be happy, but as the only morph that naturally feels emotion that's a future he can't give them. The best he can do is give them a peaceful rest in death by bringing a premature end to their fate as tools of war.

And even that horrible excuse of a 'good ending' is something he can't bring about himself. With no combat skills, he has no hope of standing against morphs created for war, and with no words, he can never hope to convey his wish to those able (or willing) to fulfill it. He must instead give those who would stand against Nergal a nudge in the right direction.

He first appears in chapter 19x, where he seals the magic of Aion (the map boss), helping the party avoid having to face his formidable magic prowess. Aion gets pissy and calls him 'that good-for-nothing', clearly indicating that Nergal's forces are at least passingly familiar with Kishuna, and I think it's a very easy leap in logic to assume that this is because he's been a thorn in their side for a while. Why else would Aion, who is obsessed with Nergal's greatness, instantly recognize what Nergal considers to be his greatest failure? Because Kishuna has done this before, perhaps even to Aion himself.

23x is the next time he appears, and Pent provides a tiny bit of context for what a magic seal is (really he just tells us that it's an actual thing, and has a name, which... sure. he's mage general, not mage 'specialized knowledge of freaks of nature'). The party speculates that he's summoning the morphs in the ruins to fight back (and that he had even "created this place"), but here's the thing: in every encounter, even though he always comes as a package deal with some morphs that are ready and raring to go, Kishuna himself never defends himself. Surely, if he wanted to, he could at least flail about with a sword or dagger or something, right?

If we proceed under the assumption that his end goal is the extinction of all morphs (which also entails Nergal's eventual defeat since he'd just keep making more), it's almost effortless to reframe him summoning morphs to fight in every encounter to him leading his siblings to slaughter. It's unclear exactly how he tells morphs where to go (also unclear how anyone else does, for that matter), but that leads me to talking about my next vision of the truth assumption.

The places that Kishuna shows up are either old workshops of Nergal's (23x and 32x) or otherwise places where he stores morphs when they're not deployed (19x, though I think the building Aion is standing at is probably a workshop). Nergal would have commanded them to fight back should they be besieged, and, recognizing Kishuna as their kin and thus misidentifying him as an ally, they only attack the humans that approach. In all three cases shown, this group of humans ends up being the party, though I wouldn't rule out Kishuna directing other groups in the past or even simultaneously (he's shown to be able to warp, and while it's unclear how that works due to the whole magic seal thing, it is pretty undeniably something he is capable of, so travel time is nowhere close to being a problem).

Going back to 23x for a moment, while Hector merely muses that "it didn't seem human", at the end of Eliwood's version of the chapter (22x) we see another reason why Kishuna has to resort to luring people to these locations:

Pent: “The air… It’s returned to normal! The magic seal has fled.”

Eliwood: “What in the name of all that is decent is that? It has human form, but it’s… It’s not human!”

Pent: “……”

Hawkeye: “……”

Ninian: “…That was a creature of heresy. Its existence makes a ruin of nature’s most basic laws.”

Eliwood: “Ninian?”

Ninian: “…Ah, Lord Eliwood. I…”

Pent: “…In this world, there are many mysterious things that fall beyond our realm of understanding. Come, let us leave this place. A living legend awaits us.”

The mere presence of Kishuna being nearby is enough to make even the normally soft-spoken Ninian reject his humanity. Ignore that he's not human for a moment; he's still clearly a person from our perspective, but nobody in the party is willing to ever entertain that he's anything more than a mere creature, a beast that upends the natural order and can only bite when provoked. Aion, scornful man that he is, at least acknowledges that Kishuna is capable of intent, but if the people Kishuna wishes to ally himself with can only see him as a pest to be exterminated, what option does he have but to lead them around? To use his ability as a magic seal, which can act as a signal that permeates walls and spreads across battlefields, to beckon them towards that which must be destroyed? To use their primal sense of disgust towards his mere existence to his advantage?

Rejected by your father, ignored by your siblings (though it is by no fault of theirs), and deemed unworthy of life by everyone else... how unbearably lonely that must be.

32x is Kishuna's final appearance. It goes about the same as all the others. He appears, you kill morphs on the way to him, and then you fight him. The only difference is that this time he doesn't run away. He simply sits there, on his throne, and awaits death.

He had done all that he could. Perhaps he could instead flee and return to seal Nergal's magic but... would our heroes be able to strike Nergal in that moment? Or would his presence muddy the waters, distracting them long enough for Nergal to regain the upper hand?

He couldn't risk it. This was the closest anyone had gotten to stopping Nergal, and if he blew it now, who knew what future would come to pass?

It was time. He stood resolute, read for the blades that would come down on him. He had prepared centuries for this moment, and he couldn't let any nagging sense of self-preservation get in his way.

And yet...

As his body was pierced by swords and sabers and spears, he couldn't help but wonder.

Could things have been any different?

Was there an ending for him, an ending for his siblings that didn't hurt so much?

With his last breath, as his body crumbled to dust, he found the strength to ask himself one final question:

Were he and Nergal destined to be opposed until the bitter end, or could they have found a different path? Was there an ending for him, his siblings, and his father that wasn't so miserable? Or were they always going to be doomed to this fate of conflict, war, and annihilation?