I just had this scary theory not related to this chapter... what if the mysterious boy that tells Uichi to commit suicide is NOT the same as the "middle school betrayer". Uichi may think they are the same but Clover likely could just find some way to hijack the middle school betrayer's phone address and text app for their own purpose after that student was gone. Clover is crafty. The reason I say this is the possibility the middle school betrayer (Rei) killed himself and blamed it on Uichi. I can't really say evidence without going into future chapters that haven't been translated but I will say some basic things.
The way the mysterious boy looks kinda looks different from Rei. His hair is more spiky and his eyes are beady. Consider how little the other characters change between their different ages and it stands out.
The way the mysterious boy acts seems odd for someone who wants to kill Uichi. He almost seems distant, as if he does not care about Uichi and is doing some job. A later chapter shows him almost focus more on another character than Uichi.
Also consider something horrific must have happened involving Rei for Uichi to not want to mention such a "grudgeful person" and for others to want him dead. (Yes Eito is an unreliable narrator and sadistic beast, but notice how his behavior seemed to only turn for the worser after the incident happened and enabled Eito to escalate his behavior without consequence, didn't seem like the intensity increased out of nowhere).
To summarize the theory, I believe Rei killed himself which is why Uichi was tortured in middleschool, the texting boy is a red herring look alike that is either Clover or working with Clover. Regardless, "Rei" takes over the deceased student's text to convince Uichi to kill himself, and Uichi doesn't bring it up cause he was too depressed and willing to take the advice to care anymore.