Endou Yuu is a fairly minor part, with their only starring chapter being chapter 00. I vaguely remember him in flashback sequences later on in the comic, or nightmare vision like moments where he might have more lines, but he's mostly absent. He's a Student that Ren meets on his first day at a new school, and who almost becomes Ren's first friend there, until an unfortunate fall.
https://www.boywhofell.com/comic/ch00p00
Here's the chapter he's in, in case you want to read it to get an idea of who he is.