Those stories are very interesting! I especially liked the "Mariah Carey Rainbow --> Ayumi RAINBOW" one xD
For me: (I'll spoiler the standard ones)
Spoiler:
Utada Hikaru: like everyone else I heard Simple & Clean is liked it. Then I discovered "COLORS" and it was love at first sight. That might've probably been the first J-pop song I've ever listened to! Then by accident, I found her EXODUS album and bought it, which made me into a true fan xD
Ayumi Hamasaki: can't really remember, but she's the second Japanese artist I've listened to. My first song was evolution, and just as in Hikki's case, love at first sight! That was during the Secret era. First CD I bought of her was GUILTY at release.
Koda Kumi: Final Fantasy X-2, I looooved real emotion and 1000 words, and then I found out Kumi was the original artist! I really hated her versions though, I just don't like her voice. I've learned to appreciate her now though (thanks to TABOO

) but I'm not a fan.
Shibasaki Kou: One of my favorite Japanese movies is "Chakushin Ari" (One Missed Call) and she plays the main part. Also, I loved the song that plays at the end of that movie (Ikutsuka no sora) so when I found out that's also Kou.. I loved her xD but I'm not a fan.