tbh it’s just a theory right now, no one really knows loll but it’d be such a juicyy angsty theory if it was his brother.
it could be him or not, in ch15 when they showed his backside his hair color was on the lighter side. and when they showed the crush in chapter 10, he had dark hair. so maybe he dyed it or it’s just a completely different dude lol.
either way, jaeyi mentions his brother always takes what doesn’t belong to him, so i’m sensing some jealousy in future chapters

the concept of jaeyi having taken all of ideum’s firsts, but he’ll never be ideum’s first crush/love because that belongs to his older brother asdfkgjsjakdj