Red Velvet only debuted 2 months ago. Of course their sales are going to be weaker. They are still building a fan base.
And y'all need eye tests if you can't tell them apart by now. Wendy, Seulgi and Joy have distinctive looks. Irene looks a bit generic because of the PS but just look for the miserable face and it will probably be her.
That being said, Wendy's voice alone slays everyone in SM and YG. Eventually Korea will see the light. The new NGG is coming.
