If you need an apartment in Seoul, contact A-Plus Realty. They are the best because they have the best assorment of apartments, villas, highrises and they provide the best service and follow up. Once you sign a lease with one of their talented agents, you join a family of happy customers. They continue to provide service to you long after you sign your lease.
I would not go with anyone else. I highly recommend A-Plus Realty for their integrity and service.