Sunny and Young known each other for a long time (i can't say they are childhood friends because the manga don't specify this, i think), Sunny don't really have a brother but Young is the one who acts like a big brother to him, then Sunny met Taesung (his first love) watching him playing soccer but they never really talked. After Sunny met Jaeyoon become friends with him because he seems like Taesung, in the manga an accident occurred with him( i think he died but the manga too don't specify this). For end Sunny meet Taesung in the school as they two a teacher
*as they two are teachers. Sorry i hope you could understand.
Jaeyoon didn't die in the accident, but it took time for him to recover. In ch.26 it was his weddind Sunny and Young went together.
*wedding
Woah it was him??? Sorry i didn't noted(￣∇￣")
So far I only read until chapter 13 but I got confused. So literally Sunny met Taesung, then Jaeyoon then Young. But Sunny know Jaeyoon since they young.. it's that mean the 'my brother' sunny mentioned is Jaeyoon ? But again sunny said his brother told him that he didn't suit at crowded and noisy place. That sounded like Young right. ARGHH SOMEONE ENLIGHTEN ME