maybe its a weird spelling choice as seon-o? weeyee
I double checked both the korean raws and novel. It's Seon-oh lol. The water ghost called him like this "선...오...야..." which means "Seon...Oh....yah...". Idk why they put it Seo-noh which is just simply wrong.
so let's me kiss it please ε=ε=(ノ≧∇≦)ノ