I have a very whatadoodle theory. Originally I was thinking Yotsuha was the mastermind behind Uichi's downfall. There are a lot of little things that give me that impression, but the two that stand out are the profile name on "his" private account- @fourleafcloverlv, and the icon for the person feeding Yamato advice has a clover icon. It isn't much, but Yotsuha's clover motif is too strong not to make the connection. Also, it's really weird that a totally random stranger would spend several years helping Yamato ensure Uichi gets bullied, AND uses her imagery. It could be someone we haven't met yet who also has a crush on her, or it's her.
But as I was reviewing the manga, I noticed another, more subtle theme. Time. We're introduced to Yotsuha watch first. In almost every scene, the watch gets featured at least once. At the end of chapter 1, we see presumably Uichi in a casket, from Yotsuha's perspective. Chapter 2, she knows what Uichi's mom is about to do and takes steps to keep him away while humbling her. No matter how much of a mastermind she is, there's no way to anticipate that, right?
Chapter 3, she knows exactly where to put the toy, to make the bully Eito fall onto the tracks. Chapter 4, she gives him a clover ring that happens to protect his hand from the razor blade in chapter 5. Chapter 4 again, she lets slip that she knows exactly how bad the bullying has been. Chapter 6, it's either a mega manga coincidence, or she knows he's having a terrible nightmare and calls him to break through it. That same chapter, she has a nightmare about him hanging himself. Could be an expression of her 'dere level of worry, or it could be her reflecting on something she's seen. The chapter end text is telling, though. "It will never heal. Tears forever locked away."
Chapter 7, she instantly clenches up when Yamato arrives. She grabs his wrist when he was doing the anime dismissive handwave thing, thinking he was going to attack. She knows what he's about already. Chapter 9, she thinks "Let's give it a try" before running into a senior who gives her the book that helps to prove that Yamato was behind the bullying. And she flips through it and happens to land on the exact place where it can be proved. Same chapter, she recalls a bloody baseball bat. But we've never seen anything like that, and certainly not within her frame of reference. Closest is that video of him bloodied and prostrating, but she has no reason to know about that.
Chapter 10, she sabotages the door to the storage room, knowing Satsuki would go there in Chapter 11, despite them having never met previously. Chapter 11, she tells Uichi she's gotten really good at making friends, but she hasn't made any yet. Chapter 13, she already knows that Yamato is being fed instructions, and plants the seeds for others to think about that. Also, she just happens to be carrying a photo that proves he must be the bully. Chapter 14, she thinks to herself that "we won't get an explosion tomorrow" like she was avoiding a flag.
Finally, back to the watch. It isn't the only time clocks show up. Nearly every chapter, there is a clock in frame, either directly or in the background. Time is a theme for this manga as much as clovers are.
So with all that evidence, I believe that Yotsuha is in some kind of Re:Zero style time loop, trying to save Uichi from dying.