Okay, now I understand!
I find jpop groups waaaaay different than kpop groups. I mainly like Japanese solo artists more, but there are some groups I like. Here are some the stuff I have on the phone (the stuff I found.. Too many songs >_<)
Supernova (Choshinsei - Japanese name/Choshinsung - Korean name) - Korean group who are most active in Japan. I really like their songs.
http://www.youtube.com/watch?v=xmmdRPprhEs
(Their latest Japanese single)
V6
http://www.youtube.com/watch?v=MvQetW6IyzY
(I couldn't find better songs on youtube ><... But I like this song too x) seeEKSHI!)
Morning Musume
http://www.youtube.com/watch?v=WzvwlrLkQIs
Puretty (They're Korean, but they've only debuted in Japan, I think)
http://www.youtube.com/watch?v=iOgeZyFunqo
Tohoshinki/TVXQ (From when they were still 5, I like their Japanese stuff the best, actually. I haven't made up my mind about which one I like the best now)
Kat-Tun (already mentioned, and you like them.. Yay!:D)
I also like News, but I don't know.. My favorite song by them is REEALLY Japanese flavored:P
I feel that jpop groups are more Japanese flavored than Japanese solo artists. If you want, I can post some songs by solo artists I think are cool?