In Kimi no tame nara Shineru, having these following GBA carts in the GBA slot of the DS unlocks clothing and wigs for characters in the game.
Software title --- Parts unlocked
Sonic Advance --- Sonic
Sonic Advance 2 --- Chao
Sonic Advance 3 --- Ulala
Puyo Puyo Fever --- Puyo Puyo
Sonic Pinball Party --- Nights
http://game-science.com/news/000865.html
Software title --- Parts unlocked
Sonic Advance --- Sonic
Sonic Advance 2 --- Chao
Sonic Advance 3 --- Ulala
Puyo Puyo Fever --- Puyo Puyo
Sonic Pinball Party --- Nights
http://game-science.com/news/000865.html