Welcome to CAA.
Just saw this thread, so I hope you're still around to hear the reply.
I don't listen to K-pop myself, but I can recommend a few J-pop artists.
#1 KOKIA: KOKIA is a Christian J-pop singer. That shows up especially in her earlier albums, where she has some songs about wanting to use her voice to glorify God (I find these a bit preachy myself, but I'm sure they come from a good intent). However, even in her other songs in which she isn't mentioning anything overtly religious, her lyrics are clean and she expresses some good values. She's also a classically-trained singer with a beautiful voice.
#2 Sakamoto Maaya: I've never read anything explicitly saying that Sakamoto Maaya is a Christian, but some of her spiritually-oriented lyrics really make me wonder whether she is, or at least has an interest in and is sympathetic to Christianity. (Particularly "Melt the snow in me" from the mini-album Driving in the Silence, a song which is sung in English, by the way.) Like KOKIA, she has very clean lyrics that express positive values and have good messages. Besides that, she has a very pure, beautiful voice and has done a lot of collaborations with the famous composer Kanno Yoko (so, she has a lot of really well-written songs).
#3 Tamaki Nami: Reviews suggest that the album Don't Stay and those prior to it are better than her more recent ones, but I mention her because she has a lot of lyrics that are just very positive and encouraging. She tends to sing a lot of upbeat dance tunes. "Heroine" is a good example of the kind of encouraging, positive lyrics that appear in a lot of her songs.