An interesting thing to consider when regarding Seteth’s early suspicion of Byleth as an unvetted, unqualified stranger being appointed to a position of authority over other, more established candidates is that, once upon a time, he was in exactly the same boat as her: an unvetted, likely unqualified stranger being appointed to a position of high authority over other, more established candidates.

For one thing, the circumstances of Seteth’s coming to live in the monastery must have been really chaotic, emotionally. According to the wiki, he joined the monastery in the same year that Byleth was born, so regardless of the exact circumstances, if he finally couldn’t take the isolation of living in Zanado any longer, or if Rhea, distraught over the apparent destruction of their mother’s Crest Stone, asked her one family member who was A) still capable of assuming human form, and B) awake to come live with her, Seteth comes to live and work in Garreg Mach, and Rhea is obviously distraught about something, but won’t tell him what. Seteth being the kind of person that he is, when she won’t tell him what’s going on, he starts trying to dig up information about what’s happened recently, hoping that maybe he’ll get answers that way. But the only thing he can find that might be connected to Rhea’s current behavior is that not long back, the captain of the Knights of Seiros deserted his post and hasn’t been seen since, and that not long before that, there was a fire in one of the buildings on the grounds that claimed the life of the captain’s infant daughter. And Rhea continues to stonewall him, because she almost certainly doesn’t want to go anywhere near the topic of the experiments she’s been carrying out, but Seteth doesn’t know why she’s stonewalling him, and is left to draw his own conclusions.

(My suspicion is that, before she discovers the truth of Byleth’s parentage and why she is the way that she is, Flayn is operating on the assumption that Byleth is Rhea’s daughter, because Flayn is nobody’s fool and she knows that bright green hair and eyes = Nabatean, but she doesn’t have all of the relevant information yet. I wonder, sometimes, if Seteth didn’t harbor similar suspicions for a time about the exact nature of Rhea’s relationship with Jeralt. To him, it was likely always pretty far-fetched; he doesn’t remember his sister as ever having been fond enough of humans to be willing to have a child with one. But the timing of Jeralt’s daughter’s death, Jeralt’s desertion, and Rhea having something close to a nervous breakdown probably paints a specific, inaccurate picture to someone who doesn’t have all of the relevant information. Couple that with the fact that Jeralt’s obviously a first-generation Crest bearer to someone who knows what to look for—hasn’t aged a day in over twenty years, and there are probably records relating to his service to the Church that go very far back—and the fact that Rhea is ostensibly way too willing to just let bygones be bygones when Jeralt resurfaces, and I think Seteth could be forgiven for any suspicions swirling at the back of his mind.)

But then, he’s appointed to the position of advisor to the archbishop, and that’s where the fun begins.

Because Seteth is, as I said above, a complete stranger being promoted to this position over a bunch of well-established church officials who likely did not take that lying down. “Who is this person?” they keep asking of Rhea, and Rhea won’t give any of them a straight answer. “Where did he come from?” they ask. “There’s no record of him serving in any church anywhere in Fódlan.” And still, Rhea gives no answer, and Seteth can only hedge and evade, which makes the well-established church officials even more suspicious of him than they already were.

So Seteth’s suddenly in charge of a bunch of subordinates who resent his advancement over them and are suspicious of his origins and why he’s been given this position in the first place. Probably even the officials—whoever they are—who were given Rhea’s blood and/or slivers of her Crest Stone don’t know exactly who Seteth is for reasons of safety, and don’t know what to make of his appointment at all. They trust Rhea, but they neither know nor trust this stranger who has suddenly come among them, and at first, they probably respond to Seteth just as Seteth responded to Jeralt and Byleth. They trust Rhea, but they fear that whatever relationship she has with this stranger may be clouding her judgment.

And just how qualified is Seteth for this job, starting out? In the intervening centuries between the battle of the Tailteann Plains and Seteth’s coming to Garreg Mach, there is no evidence to suggest that he did much of anything beyond watch over Flayn’s resting place. Any administrative skills he might have had prior to his going into seclusion must certainly have rusted in the well over nine hundred years since he last really put them to use. The fact that the technology hasn’t changed at all since he was last a part of the world (thanks, Rhea) might help keep him from being completely useless at first, but it is likely woefully obvious the amount of catch-up he’s having to do just to figure out how to do his job. So yeah, resentment and suspicion just deepens.

Oh, sure, eventually, Seteth earns the respect of the people he works with. Part of that is the fact that he does do the work to figure out how to do his job well. Part of that is the fact that many of the people who were suspicious of him when he first entered the monastery have since retired and/or died by the time the game starts. But the road to getting there is rocky. Littered with passive-aggressive backbiting, definitely. Littered with unsavory rumors regarding the exact nature of his relationship with the archbishop, maybe.

And he has likely never forgotten exactly what his work life was like when he first started working at Garreg Mach, so when Byleth shows up and is appointed to a position of authority to which she is not at all well-suited or qualified?

Well, I think Seteth is finally understanding, intimately, what many of his own critics were thinking when they picked at him all those years ago.

I think he’s also looking at Rhea and saying “You seriously want to go through this again?”

And he understands just how nerve-wracking it is to have someone he trusts making these seemingly inexplicable decisions, and her refusing to confide her reasoning in him all the while.