the word you're looking for is Backpfeifengesicht, which is wonderfully descriptive and generally applicable every time Kai asks that question. Tho I do find him pretty fun, so I don't mind it quite so much :D

I think there's also definitely an element of... it's been a while since our major antagonistic force hasn't been a Rider? With the exception of Hibiki, every season since Ryuki has had another Rider be a major antagonistic force in some sense, so Kai despite his weirdness feels far more 'traditional' than Kaixa or Dark Kabuto.

I just love the idea of Ryotaro, who is not great at showing emotion, having an opposite number who can't tell what emotion he's showing. I love that twist so much.

But, yeah, Den-Parado is a dude who feels like a legitimate culmination of this story, even if it's not exactly the direction the show seemed like it was going in. (The idea of someone being in charge of the Imagin literally never comes up once until Kai appears! It's not even really teased except possibly for Ryutaros's debut, if memory serves.) He's saying things about emotional stability, about consequences, about memory, and he's doing it all with a physical presence that can both threaten and be threatened. I like what he's adding to this show!

...unless what he's adding is the new version of the theme song, which I hate.

KAMEN RIDER DEN-O EPISODE 40 - “CHANGE IMAGIN WORLD”

https://kamenriderdie.com/images/kr/deno/den40a.png
https://kamenriderdie.com/images/kr/deno/den40b.png

Kai! Kai.

A serious villain could never work on Den-O, but a Kamen Rider finale needs a serious villain. The contradiction of that, finding a resolution to it… maybe the best idea Kobayashi has ever had.

Kai is a hilarious villain who thinks he’s serious, doing horrible things in the most lackadaisical way. He’s ruthless in pursuing his goals, but willing to have a conversation about why he’s pursuing them. He’s cunning in his strategies but generous in allowing opposition. (That look on his face when Deneb scans him for a ticket! And Kai just lets him!) He’s a villain that, on a long enough timeline, you could absolutely see joining the cast as a hero. He’s got the same limitless charisma that Ryotaro’s actor has, but calibrated differently.

It’s his weird sense of camaraderie. Kai, Ryotaro, Yuuto, they’re just a group of guys caught up in this weird scenario, no different than someone you’d sit next to on a long train ride, or a co-worker. In fact, Kai totally treats Ryotaro like a co-worker in this episode. Their scenes together, the energy in them, you’ll recognize it if you’ve ever hung out after work with folks, gotten a drink or something to eat. The shittalking, the shared boredom, the gossiping. Kai’s whole pitch to Ryotaro is what every late-night after-work bitch session revolves around: Let’s Laugh At How Stupid Our Job Is. He’s a villain that thinks this is all pointless, all subjective, all irrelevant. The idea of caring about what happened to Yuuto is baffling to him, hilarious. He’s trying to prove a point to Ryotaro that caring about any of this, trying to save people or mourn them or prevent Kai’s plan, it’s stupid. It’s a villain who is attacking self-determination and empathy with moral relativism and apathy. Kai is the absolute best villain this show could have.

And Ryotaro rises to the challenge in this incredibly weird episode. Like, the show is not subtle with its thesis statement here. The new timeline is objectively better without Yuuto being in it. Literally everyone is living a happier life. (Well, probably not those two women with Ozaki. I don’t know them, but they can do better!) Every single person is better off… but that would mean giving up on Yuuto, and Ryotaro can’t abide by that. The idea of trading lives is abhorrent to him, inconceivable. Yuuto did exist, he is remembered, and Ryotaro will work to save him. That’s it. It’s an argument grounded entirely in faith, in the belief that no one deserves to be forgotten. The suggestion that the world is better without someone in it has at its core a flaw that Ryotaro can’t overlook: what about the person being forgotten? How would they feel? As long as the argument assumes that’s not worthy of consideration, you’ve lost Ryotaro’s support. He’s meeting Kai’s What Aboutisms with certainty, conviction. Yuuto lived, and he deserves to be remembered.

It makes for an episode that’s largely just that dilemma, and Ryotaro’s feelings about it. It’s nothing that needs him to work at solving it. The timeline ends up reasserting itself, putting everything back where it was an episode ago. There are no real consequences to Yuuto getting killed, since Yuuto just pops back up. It’s a weirdly circular episode, for how it’s Kai going Memories Are Dumb and Ryotaro going Memories Are Actually Great and that’s sort of it. Really great performances, and some well-shot action, but it all works more as a thought experiment than a dynamically plotted episode of tokusatsu. Clever, but maybe too clever? I don’t know. Kai’s great in this, Ryotaro is an awesome Den-O, that’s plenty for one episode.

https://kamenriderdie.com/images/kr/deno/den40c.png

THE BAGGAGE CAR

-Hey, you’re right, I didn’t talk about Zeronos’s new Cosmic Rust Form! Beautifully designed sequence, and probably a crucial piece of this story’s puzzle… except, AS PER GODDAMN USUAL, I’m not sure exactly what happened in this Yuuto development. He’s burning up a new memory to power up Zeronos, but I can’t tell from this montage if he’s burning up his memory of Airi, Airi’s memory of him, or both of their memories of each other. That matters! It matters a whole lot! I wish this episode nailed it down more, but I feel like it’ll get elaborated on next time. (That is basically how every major Yuuto storyline works.) Meanwhile, I thought it was really pretty? I just don’t know what it means yet.

-I cannot even begin to tell you how much I hated this episode’s theme song. I guess it’s the DenLiner Imagin singing parts of it now? It’s horrible. It’s menacing and brash in a way that is antithetical to this show’s vibe, and I hope it was just some one-off goof for this alternate reality. Listening to the theme song was always a delight, and this was like nails on a chalkboard to me.

-Deneb on the DenLiner as a cook is fantastic. Deneb as, like, Den-O’s assistant manager is the best. He’s so good at his job in literally every timeline!

I'm happy you're getting so much enjoyment out of Kai, and am also still struck by how apt that fog bank metaphor you used to compare our reactions to the Zeronos stuff was:

This episode was also directed by our old friend Naoki Tamura (TV Asahi), by the way. Dude really did some great work on this franchise.

But, hey, for once, we're totally on the same page about one thing in Den-O:

...so that's something. :p

Yeah... yeah. I don't know! It felt devastating. Everything about that sequence is horrible, everything about it feels like Ryotaro is letting Yuuto down by allowing this to transpire. It's Yuuto opting for emotional turmoil, and here's a fun Deneb Gun to go along with it.

I just don't understand the stakes, and that kneecaps it for me. Someone is being lessened by this, and I don't know who yet!

Also, that new theme song is a war crime.

Airi is the oblivious type of the "harem" (... Airimirers) she got. But she's the inverse of what usually happens in media where harem had the target of interest being a guy, with them surrounded by numerous girls, and the guy mostly or never notice about their affection to him. Likely done for meta reasons as well to freely ship him with any member of the harem or freely have him interact closely with any girls without painting it as cheating.

I guess that low level Imagin is also fought in the past if Momotaros can possess Ryotaro without much problem and even getting overexcited for that. And Imagins are mostly fought in the past, that'd mean, although you finally appreciate Liner Form's motif and meaning, the Taros Imagins' struggle against Ryotaro would be negated for a good number of times. Momotaros may have a blunder here, but that's how to teach Ryotaro how to fight, by possessing his body and have Ryotaro feel directly the movements for him to be used as a hint. It seems not only Hana/Kohana, Owner also seemingly have superhuman capabilities, but both has never utilized them to save the day. And for you considering Yuuto kicking an empty bottle that it hits Ryotaro in the face, that's Ryotaro's gimmick as unlucky loser, but you only mention it now?

At best, S̶h̶o̶u̶m̶a̶ Kai operates on the end justifies the means mindset (which is one of the dark triads in personality), to only focuses on (what he thinks) the better outcome, regardless of how many victims and bodycounts that he'll eat in his scheme. But then again, it was all talk here, it was merely a lead-up to killing Yuuto and trying to trick Ryotaro into not interfering by showing him that it made for a better present. And for tolerating the women equivalent of Airi's customers instead of the Airimirers, I smell some agenda here... though this one's may be a joke too.

Well, for you to have this thought of Ryotaro having this view of Airi, to have her no longer tortured, do you accept this in his perceptive or not? Because, not that I like this trope, but the keeping the harsh truth from someone is done for that, to make that someone not feel tortured or conflicted?

Well, for the very least, there's a good reason of why Ryutaros is so scared of him, although Ryutaros is someone on protagonist side, and the failure is about him not killing Ryotaro. But for the other Imagins, well, the one who killed young Yuuto here is S̶h̶o̶u̶m̶a̶ Kai's underling.

Being far ahead of the heroes means you don't suck in the threatening part. But that'd mean suck for S̶h̶o̶u̶m̶a̶ Kai's victims (if other Imagins are this), that threatening would mean others are in danger. S̶h̶o̶u̶m̶a̶ Kai may show none-of-this-matters-to-him thing... for now, doesn't mean forever. Remember your words for what you said above for S̶h̶o̶u̶m̶a̶ Kai being seen as a pushy dick to his underlings regarding that.

I think it's the other way around: Momo can only possess Ryotaro in the present, which is where this fight is taking place. The Bunny Imagin is trying to widen this dude's Time Hole, but Den-O stops him. Ryotaro doesn't go to the past until the climax of this episode.

This time it was exceptionally funny?

So, no, I don't think Ryotaro ends up being right in this story. The idea that Yuuto's variable existence is worth everyone else's misery, I don't agree with that. (Especially since Yuuto's purging the universe of himself anyway!)

But, first, the show is making a point about the steadfastness of Ryotaro's beliefs, not whether he's always right or wrong. Second, this isn't a situation where the truth is being kept from Airi. This is a situation where Ryotaro would be (theoretically) rewriting time to depress Airi. Not a great choice for him to make!

Yeah Climax Jump Denliner form is definitely inferior to original in every way and I hate that it replaces it for pretty much the rest of show I think.

Since we're talking about Climax Jump I should mention their are a couple different variations of the song sung by the different Imagin:
Sword Form(Momotaros):
https://www.youtube.com/watch?v=ljbEkZ9yJio
Rod Form(Urataros):
https://www.youtube.com/watch?v=H6jO2yfW3vg
Axe Form(Kintaros):
https://www.youtube.com/watch?v=jcwxD_aYdTg
Gun Form(Ryutaros):
https://www.youtube.com/watch?v=NgewKMpsHfY

Bonus Information
Their is a variation of Double Action sung by Naomi and Airi called Double Action Coffee Form:
https://www.youtube.com/watch?v=qYka6jJ-juI

Yeah, these timeline changes! Some of them are for the worse!

(Also, thanks for those other songs! The Airi/Naomi one is maybe my favorite version of Double Action!)