Please skip this post if you don't want spoilers for the Keyframes VN
BLIND YOURSELVES BECAUSE I DON'T KNOW HOW TO ADD THE BELOW THE LINE FORMAT, I'M SORRY Y'ALL
Okay, hopefully that was enough, because if it wasn't, I'm so sorry.
Okay, given that the devs confirmed there's one LI that will only have routes that lead to break-ups, I've thought about this for a while, and I think most people agree with me on this: It's Percy. Now, do I have concrete evidence for this? No, but hear me out.
Percy is probably the least straightforward of the LIs. It's easy to tone him down to the funny, cute, gamer guy, but I think everyone in this fandom has too much media literacy to actually believe that. A lot of Percy's key moments are found within noticing the little things. The change of his expressions, the change in his speech, etc. We also know that Percy probably had a complicated childhood and adolescence from a few of the asks. He's also the only one of the LIs that has actually dated.
I recently saw a post by @jadejasiones saying "Percy is the type of guy that's so stuck in the past that he's even experiencing his present in the past."
That really stuck with me because it feels totally in character. We know that Percy has a stepdad because it's said directly in the game. I am unaware of at the moment if it was because of divorce or because his mom/other parent remarried after death, but either way, divorce/death can really traumatise a child and can often leave them wishing for the past. "I miss when mom and dad were together," "I miss when dad and I could cook together every weekend," they long for the life they had before said traumatic event.
From this, I think he experiences anticipatory grief: where he expects things to end or go wrong, believes that he will mourn again for this great thing that he has at the moment. We see this in how Percy interacts with the MC, even if MC played along and might've even flirted with him. He expects this totally fun, upbeat interaction to have soured his impression on this seemingly reciprocating stranger.
I am increasingly blanking out on all the times I played his route, so I can't really remember if there was anything else, but even in the Spring Demo, where he's essentially a stranger, there are still moments where you can take a deeper look into his behaviour. I think Percy might end things with MC for one of two reasons. 1. Because MC isn't actively maintaining their relationship. 2. Because things are going TOO well.
The latter could be caused by multiple things. The first being his self-doubt or self-deprecation, which we can kind of infer from the demo from multiple occasions. (Snack run, rain scene, etc.) The other being from wanting to cherish this perfect moment, the perfect time he has with you/MC. He would mourn the very thing he put an end to. He would probably suffer quite a bit even though he chose to break it off. This anticipatory grief was a self-fulfilling prophecy.
I don't think it's Jamie or Elio for a few reasons.
Elio is a lover boy. He watches the old movies with dancing in the rain, he dreams of having a partner and a family, but would also give that dream up if his partner didn't want it. This man doesn't fear commitment at all. I truly think there's only one way this man would break up with MC. (Beyond losing feelings.) That being, MC is not holding up to their side of the relationship. Unless the devs surprise me terribly, I think Elio is the least likely to pull away.
Now, it makes sense for Jamie to pull away, yes? He's the mysterious "ice-prince" who hasn't fully let down his guard for anyone. Well, no. I think he might be the more complicated LI to get into a relationship with and his definition of a satisfactory relationship might differ from others, but Jamie isn't nearly as inexpressive as most people in the game might say. It's pretty easy to tell when he's not comfortable or doesn't want MC to do something. I think the key to a relationship with Jamie is clear boundaries and honesty. Once he feels safe, I highly doubt he'd go out of his way to sabotage himself. There's two ways I can see Jamie breaking up with MC: The first being that MC isn't pulling their weight in the relationship/violating his boundaries. The second being him losing feelings, though I feel that nothing has alluded to that so far. It's also been confirmed that Jamie is not moving back to Sweden. (at least for the events of Keyframes.)
Though, this is a very premature hypothesis; we don't know that much about the LIs and I could be proven very wrong within the next few years. We know a lot about their lore if you dig through the asks, but their day-to-day life, their full history, all their quirks, and most of the things we'll learn in the game will only come with time. So, let this be a marker of some sort to see if I'm right.