Just because someone is a Potential unreliable narrator doesn’t mean they necessarily Are. Especially if we’re playing them in the moment as opposed to them relating a story Afterwards.
And it’s also a sitcom trope to fall for a coworker and think it’s more serious than it ends up being.
¯_(ツ)_/¯
It doesn’t necessarily have to be an unreliable narrator, MC just doesn’t know everything about R.
I think so too. It’s probably going to end up Rainn sleeping with this person only to realize the mc is the one they really love. Or the stranger end up breaking up with Rainn letting them feel how the mc feel about the break up and then realizing what they’ve done. There must be a variable for Rainn wanting to go back and I feel like it’s this person. This is pretty much common in sitcom.
They might not even sleep together. It might just be the proximity of MC, paired with her realizing the feelings she (potentially) has for this other person are just some work stress things.
And you know what ALWAYS makes people appreciate their Ex and all the good times?
Seeing them happy and productive.
And I’m sure living with three attractive members of the opposite sex in the same building won’t hurt to spur some feelings of… Possessiveness. 
It’s quite interesting to speculate about who R is embracing and giving a lingering look to. In my opinion, R’s character setup is that of a person so focused on work, like those who always prioritize perfection, that they feel they don’t have time to worry about other matters. It’s surprising that they fell in love with MC and maintained a long-distance relationship. Therefore, R is unlikely to have been unfaithful to MC during the time they’ve known each other. Do you think they have excess time and mental capacity for another person? When MC asked R if they were being unfaithful, their reaction showed they were hurt by that question and their expression was very honest.
Another point is that R has experienced trauma in the past, though it’s unclear what exactly and it’s still too soon to know. R might need to rely on the support of a therapist, a doctor they trust. And the person we suspect R of cheating on MC with is maybe just a personal therapist in the capacity of a friend, someone R feels the need to consult, perhaps due to an upcoming promotion, work pressure, or even the breakup with MC… It’s possible, as some have predicted, that this mysterious person is just a family member.
But someone with whom R is having an affair behind MC’s back? Certainly not.
I replayed it and now I think it was the person with them at the bar. I wondered why Rainn announced their break up suddenly in the bar. Even the mc was surprised caused they didn’t say anything prior, but Rainn was pretending to have said something prior. I wondered why is that. But now I get it, Rainn was pretending to be single all this time. But the mc suddenly arrived and kissed them and now they must deescalate, how? by breaking up with them on a hearing distance with that coworker. I remembered the mc said Rainn’s coworkers can hear their break up.
He could ask them to go outside, or his apartment. Anywhere was better, but in a full bar, with his lawyer bros watching the disaster. Also, it wasn’t just the lb not knowing about mc, but also cal. So what was Rainn thinking?
Edit :
As I said previously. Emotionally divesting in someone, like R was clearly doing to the MC, while being in a work situation, such as a recent promotion in a cut throat industry; can easily lead to faux feelings of attraction to a coworker whom you’re currently going through the proverbial trenches with.
It’s far more plausible than you seem to think. Not all cheating is sex and physicality. Believe me, it can far more of a blow to a person to find out someone doesn’t love or value you anymore than it does to find out they just slept with someone.
So her Never mentioning, in 3 Years, to her neighbors, friends and coworkers that she’s in a relationship with someone? Dude, that’s borderline psychopath behavior and demonstrates a fairly severe lack of personal/emotional investment to begin with.
As such, it’s Entirely believable that she went on a date with a coworker that she feels she’s grown close too; especially if she’s misinterpreted feelings if he’s making her new job easier.
A new job/promotion which she failed to even tell her boyfriend of three years about. And when asked, says “I didn’t think you’d care.”
I think I’m just gonna end my responses here so we it’s all speculative anyway until more story comes out; but Emotionally Divesting in someone and investing in another is Definitely a form of cheating. As such, Middle Finger MC from the fire escape with the hot new roomies.
I do think MC is witnessing an unreliable narrator moment. I don’t think Rainn cheated or was thinking of cheating, but I do think a lot of people are into them and they’re single now, lmao.
In general, I don’t think they were trying to hurt MC. I think they’re just a shit communicator and naturally avoidant, hence why they thought simply telling MC that they were busy and needed to cut back on things was breaking up and that any normal person would ever think that.
Exactly 
No, but Taren lowkey getting mad that MC was spending time in the apartment that they live in just because Cal also happened to be there? I wasn’t planning on romancing Cal, but I have to now just to spite them.
If this is the case. Would you still take them back even when did what they did?
This is my other devil persona 
I’m not sure how devastated the MC should really be over the breakup with R. From what we see, despite it being a relationship of 3 years, it doesn’t seem to have been very deep. I think R saying:
Rainn looks surprised until his reaction fades. “Andi told you?” You nod and he sighs deeply. “I just…I didn’t think you cared.”
is rather telling. Beyond R’s “miscommunication” about the break, this is what annoys me the most. Add to that apparently in the 3 years you’ve been dating R never once mentioned he was dating someone to C?
I feel like this is a little aggressive given that I’m just commenting on the relationship in the story. I’m not saying the MC wasn’t in love with R, just that the relationship as it is portrayed seems shallower than it may first seem.
It was deep enough for the MC to move to NY for Rainn. Deal with it, people. It’s a characteristic of our MC which we can’t choose, because the author said that it is so - we were in love with Rainn, give or take, sure, but we were and right now we are hurt. It’s the premise, it’s the thing without which there is no plot, so just suck it up, folks.
A fun fact to consider about that cliffhanger scene and appearances:
If someone (say, Rainn, but really anyone) was standing at their apartment’s balcony on late Sunday night, they’d potentially see an unknown car pull up. Then they’d see MC step out of it, and longingly gaze at their driver departing, watching them until they make a turn and disappear.
Clearly, MC has already moved on and is dating other people, right? Spending long hours with them until late night, all but ready to jump in their bed, no doubt.
Except that was just M giving you a lift.
Didn’t expect for romance to get down my throat this easily. God i love every minute of it.
While playing the Scene when Rainn dates someone, I just thought If that was like the Bosses daughter to get the Promotion or something like that, with whom they plan to break up after getting promoted and then continuing the relationship with the MC. Not that my MC will let that happen, but it Sounds as a normal sitcom Plot 
I also don’t think R is being unfaithful. It could have been a throwaway character, but I have a feeling it might be the coworker who asked R if they were okay in the bar during the breakup scene. They witnessed the breakup, they presumably already know R, it’s not unfeasible for them to comfort R in the aftermath.
Also, I’ll admit I’m mostly going off of generalisations here so if I’m being out of pocket say so, but does anyone else get a vibe that R might be on the spectrum? Hyperfocus on an interest (his law career; 12 year plan as a child). If C didn’t specifically ask about their partner I could see someone with some form of ASD never bringing it up because it “wasn’t relevant”.
Your reasoning is also fitting, but I think R leans more towards having OCPD syndrome. In any case, this is just my temporary speculation based on R’s behavior and the limited information in chapter 1, as the symptoms of OCPD can include:
A person with OCPD typically shows symptoms of perfectionism starting in their teenage years or early twenties. This can hinder their ability to complete tasks because their standards are extremely high. They may emotionally withdraw when unable to control a situation. This can interfere with their problem-solving abilities and the formation of close relationships. Other signs of OCPD may include: excessive devotion to work, reluctance to express affection…