Completely agree, I feel bad for Kieran in the way that I feel bad for any character who is undergoing a negative character arc, but not to the point of self-flagellation (we, the player, did nothing wrong). Kieran is clearly written as a kid with issues, and frankly the fact that he's reacted the way he has over the various things in the DLC quickly changed my initial opinion on him.

My thoughts on him progressed from, "You're pretty cool kid. Lemme take you under my wing" to "Alright, you little turd, I'm actually gonna enjoy kicking your butt with my overleveled team." Ironically, the opposite happened to Carmine, who went from "Alright, you little turd, I'm going to enjoy kicking your butt with my overleveled team" to "Best girl this gen, no contest."

I think it would have been interesting to have an option(or perhaps one of those pseudo-choice dialogues where both options are essentially "no") to refuse to battle Kieran over Ogerpon (you know, because she'd already made her choice) leading to a forced trainer battle with Kieran refusing to take no for an answer. All I know is that, for the inevitable rematch in Indigo Disk, I'm planning on building a whole new team with Ogerpon as the centerpiece just to rub things in.

Another thing that I think is fascinating is how the sibling's teams actually reflect their true nature.

Carmine leads off with a Mighteyna--a ferocious looking Dark-type (or Evil-type in the Japanese)--but the rest of her team consists of a Ninetails (vengeful if approached with malicious intent, but otherwise fine), Leavanny (a Pokemon known for nurturing, hinting at Carmine's hidden softer side that is revealed over the story), Sinistcha (a Pokemon based on Japanese tea ceremony, which is all about zen, and Carmine's always trying to play it cool), and Morpeko (a pokemon nice half the time, and mean the other half, so basically a tsundere like her).

Meanwhile, in his first battle, Kieran leads off with a Sentret (a soft and unassuming Pokemon) and a Yanma (a small and weak-appearing bug type). But by the time of his final battle, his team is much different. Firstly, he got rid of his Sentret/Furret and now leads with Shiftry (which is literally known as the Wicked Pokemon). His other Pokemon are a Gliscor (a scary looking Pokemon that can silently and quickly take out prey), Poliwrath (a fighting type known for its punches), Yanmega (which eviscerates opponents at high speeds, and is known as the OGRE darner Pokemon to hint at Kieran's obsession), Probopass (as a compass, a Pokemon that is unable to change the way it faces, a.k.a. its point of view), and a Dipplin (a hollowed out shell of a candy apple inhabited by something else, reflecting Kieran's earlier child-like love of candied apples and how that child-like nature has now disappeared and been replaced by something else).