So, I've been thinking about this while rewatching the first season of the 90s anime with the club...

There seems to be a sudden shift after Mamoru starts to "date" Rei where a lot sort of goes on that we don't see from his perspective. Suddenly he's hanging out at the arcade. Suddenly he's got some private thoughts about Usagi that aren't just "what a weird girl" and he actually tries to talk to her. (The one line I noticed is translated by VIZ as something like 'I guess we're not compatible after all'... I'll go look it up and correct later if needed XDD.)

Anyways, I want to say that a lot of this is probably stemming from the fact that Rei is constantly trying to get into his life and probably talks his ear off about her own life which of course has started to center around Usagi very quickly... And I guess maybe he's asking Motoki about her after the three of them realize their mutual connection... (Y'know, I have to assume he has some sort of natural interest in her and just asks about her 'cause... Well, because he's Mamoru and she's Usagi XD.) But I really wish there was more exposition of his thoughts and actions pre-Nephrite especially but really all through until their mutual identity reveal in the Starlight Tower. I feel like this off-screen development is what has given a lot of fans the impression that his character is actually personality-less in Season 1 or at least is entirely different from the personality we see develop in the later seasons. 

It's one of those "blink and you'll miss it" sort of things I think... There are a few little hints as to what's going on with Mamoru, but I feel like it would take some effort to really follow them properly instead of just leaving each episode with a vague impression that things are progressing (or not progressing >.>). I intended to go back and carefully watch each episode to note down all the small interactions they have and ones between Mamoru and their mutual acquaintances but... I can't do that and keep up with the club's rewatch atm ^^;;;. So maybe some other time or if someone else has the inclination... Also, I do wonder if the audio drama(s??) from the first season might shed a tiny bit more light on what was going on with him at the time... So much else happens in the first 35 episodes that it's hard to squeeze everything out but it'd be nice anyways XD.

I hope this all makes sense. Does it resonate with anyone at all?

Huh, I never really thought about that! Now you've got me wanting to go back and rewatch the series LOL

Oooh that's a really interesting point Dia! It's really too bad that the anime doesn't show more with Mamoru and his thoughts/intentions/motivations beyond the bare minimum that we got. There's also the whole sub-plot of him realizing he's actually Tuxedo Mask, which I believe takes him nearly half the season to realize. I guess you could argue that before that, we wouldn't get much on Mamoru since Tuxedo Mask is supposed to be mysterious until that reveal buuuuut the anime doesn't do much with him afterwards either.

An episode by episode analysis of Mamoru in the first season would be interesting but timmmmeeee. Would love to hear other Mamo fans' input too!

xD A few years ago I did a sarcastic analysis of all of Usagi and Mamoru's interactions during the first season in response to a "THERE WAS NO CHEMISTRY, MAMORU NEVER EVER LIKED HER AND "DESTINY" IS THE ONLY REASON THEY'RE TOGETHER!" anon that are still available somewhere on my tumblr xD I'd link it but I can't access tumblr on school computers, winkwonk 

I hadn't really considered Rei and Mamoru dating being a catalyst... maybe because I think their 'dating' was more from Rei's p.o.v. than Mamoru's xD Poor guy probably said yes to doing something together one time and suddenly finds himself in a "committed relationship" with the girl that hangs out with the girl that he loves to tease and he's too polite and inexperience in social interactions to find a way out of it and so is just... riding it out, hoping Rei'll eventually realize that.

Mamoru’s perspective can be a bit confusing at times, especially when it comes to romance. Though sometimes, I thought Usagi’s actions regarding romance in the first season were even more confusing at times but still quite understandable. Anyway, I just recently made a blog on Amino on an analysis of Mamoru’s and Usagi’s feelings for each other using somewhat their perspective of each other in a way. Hope that can help. .

Here’s the link below:
https://aminoapps.com/c/sailor-moon/amp/blog/my-thoughts-on-mamoru-and-usagi/eYX5_MMdI3udVa3xwNvqExdKarRMM0xGlrm