so mememi's got that promo art of the thieves all playing instruments and there's some i really love and agree with, i'm not as sold on the others. so i'm thinking.....

joker - vocals, but also guitar, bass, keyboard, drums, and a little sax. he's a jack of all trades.
ryuji - drums
ann - flute
yusuke - taiko (one of them bigass japanese drums)
makoto - bass
futaba - electronic keyboard
haru - cello......it's a big violin and you can beat people with it (also my favorite instrument)
goro - trumpet & oboe bc he's double extra like that
sumire - french horn