Let me say something, since I'm going to burst* without writing it all down. And I know I probably should take my reply to the place the discussion originated, but I don't like to confront people, so it's here in my own corner of the web.
(*Not literally, of course. But I'm the type to exaggerate my feelings like that. [Is that annoying, perhaps?]I wonder if my frequent use of hyperbole might get annoying sometimes... Being close to my own writing, I never see a problem -- sometimes I'm even a bit confused that I'm taken literally when I don't intend to be serious at all. Well, when that happens, it's probably my fault for not being clear. )
1. IIRC, I'm not the only one believing this, but seeing some comments recently stating the opposite, I just can't help saying this. Shunpei and Nitou are NOT the same type of character. At all. Nitou purposely acts funny (most of the time) -- whether his brand of humor (and/or his actor's ability to deliver it) works for the viewers is very much up for debate (works for me, though). In contrast, Shunpei is too serious for his own good most of the time (sometimes I think he is more serious than Haruto) and any comedic moment from him, imho, is his spectacular failing at whatever serious thing he is trying to do.
2. Can they be combined, though? As in, just have only one character for the 'comic relief' department, even though, to me, their ways of 'comic relief'-ing are completely different. I never really thought about it, but as it is now, I would say no. If Shunpei was built up to be a secondary rider, I think he should be on a more equal footing with Haruto from the beginning. Shunpei might have to earn his power later, but if Haruto (who is plenty serious himself) doesn't ever take him seriously, I don't think that will work. And maybe it's just me, but I find Shunpei and Haruto to be a bit too similar to play off each other all that well**. (In fact, I think it's probably easier to combine Shunpei with Haruto, if a character needs to be cut at all. They don't seem all that different to me.) I mean, Nitou and Haruto's interactions are more interesting for me to watch.
**Fine, fine. I liked their interactions in that first arc they met way back in the beginning. But mostly because of Haruto (or rather his actor)'s valiant attempt to not burst out laughing in those scenes.
3. Can Nitou's motivation be better? Yes, of course. But I'm fine with what we have. Nitou isn't a brooding character. He is a think-positive guy (danger is opportunity, remember? Though I won't blame anyone for forgetting since he never said that, like, in forever). Frankly, I find him refreshing in the sea of revenge-driven secondary riders***. Yes, I know he has no real connection to the plot and could be cut without much difficulty (only Bandai would throw up a fit anyway). But I actually like his presence. A light-hearted, irrelevant guy like him is unique (on both the hero side and the villain side) and I would even say needed. At a risk of repeating myself, Shunpei is very far from filling that role for me.
***For the record, I do like Terui.
(Honestly, Shunpei reminds me of a much clumsier version of Shinpachi (yes, the Gintama one. I don't know any other Shinpachi). Although he doesn't have half of Shinpachi's snarks. And Shinpachi has better fashion sense (though this is not an indication of anything. Anyone would have better fashion sense than Shunpei).
(Yes, I know characters like Shunpei (earnest sidekick) existed before in Heisei Kamen Rider (namely, in Faiz and Blade), but since I don't remember those characters well anymore, I'm not comparing Shunpei to them.)
Nitou, er...reminds me of quite a few people. But a recent one I remember must be Asuka from Ultraman Dyna (since I still watch that show on and off). Surprised I can't think of people in Kamen Rider to compare Nitou with (Date (from Kamen Rider OOO) might be close in term of light-heartedness, but nah. And perhaps a shade of Eiji with the free-spirit, not having a home thing****, but I don't see much similarity beyond that).
****Speaking of which, that character quirk pretty much disappears/gets dropped. I don't know where Nitou lives anymore -- whether on a street or in a proper lodging (though can't imagine any sort of living space for him that could be considered 'proper' ;).)
And, oh. Cast commentary (inspired by the above) I'm kind of shy to put on this page.