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Maybe this will help. I found some videos for the end credits screen of G1. There are others on youtube, just search "Mabinogi credits".
Anyway according to the video above...
Art Director - Rooney Yi
Character Designer - Popcorn Kang
Costume/Monster Designer - Akane Lee
Monster Designer - Nox Ahn
Scenery Designer - Sindorim Jang
Movie Designer - Ren Lee
Web Designer - Oz Yoon
Thank you so much @Adeno! I actually thought about this as well, but at the time I just searched up "Popcorn Kang" in English hoping to find something. Of course, after thinking about it, I should have known most of my findings should be in Korean!
http://mabinogi.nexon.com/m/community/kin_view.asp?id=242752
I found this thread after searching "팝콘 강마비노기" (Popcorn Kang mabinogi) The Korean spelling of the illustrator's name is probably wrong though (Used a translator, if someone is able to translate their name, let me know!)
In this thread, someone asked the same question as me. A responder posted their website link in the comments. I figured the website would be down, so I used Web Archive to see their old posts.
Edit2: I looked up another artist on the G2 credits, Kim, Jin-Hee (김진희) who drew many old works used for the Mabinogi homepage. Though I am also unable to find other works by them.