There have been co-ed KPop groups in the past, but none of them were extremely successful. I guess it depends if you think AKMU counts or not. There is a new one called K.A.R.D., and they just released their second song. The international fandom is head over heels for these kids, but it seems like the actual people of Korea don't seem to care that much. I hope they can be successful.
What I like most about them is that one of the girls originally debuted under the same company in a group called April. April is the youngest, most lolicon, creepy group I've ever seen. She was the leader, but she bailed out like NOPE. Way to go!
Their debut song
Their new song
In other news, Taeyeon, the leader of Girl's Generation, has been releasing many successful solo releases for a year or so. They are good, but they are all like ballads and such. They definitely don't make your ass move, so they don't really make the Scott playlist. Just now they released a teaser for her new song. Everyone was expecting more of the same, but the unexpected nature of this song has all the fans SHOOK. i.e.: Bodies were not ready for this.