DOOJOON

I see doojoon giving you a classic nickname like

Jagiya

HYUNSEUNG

For hyunseung it's a bit hard for me hyunseung to me doesn't seem like the type to give a nickname but maybe

My girl

If hyunseung would give you a nickname i think this would be it

JUNHYUNG

I feel like junhyung would do something simple like

Darling

YOSEOB

Yoseob would definitely give you

My sunshine

As you're nickname you'd Brighten up he's world in good times and in bad

KIKWANG

Much like yoseob you're the light in he's world During bad times so

My angel

So going to be you're nickname because kikwang would see you as an angel when something bad happens because you're always there to help him get back on he's feet

DONGWOON

Much like hyunseung i see dongwoon not being ever big on nicknames but if he is to give one he would be like hyung doojoon and give you

Jagi

Something simple