hm... hyunjoong?
as u said for significant.. i guess he's good as YS rival.
as there is JJ too as SY brother.
i prefer kyu as YS bestfriend or right hand

I would like to suggest Onew, Woohyun or Teasung (SY's parter in WGM thingy). I don't really shipp SY with guys but i have read so many fics with these guys and I would like someone who I'm familiar with and I don't mind any of them. I wanted to chose any of the other two guys but I have never read a fics with SY + any of them.