Successfully complete one of the following tasks in Arena mode to unlock the corresponding part:

CR WH01HP (R Arm): Get rank "B"
CR WB03MV (Back Unit): Get rank "A"
CR WH05RLA (L Arm): Get rank "30"
WB34M-ECHIDNA2 (Back Unit): Get rank "25"
WH08RS-FENRIR (L Arm): Get rank "20"
CR WH01SP (R Arm): Get rank "15"
WH09H-WRAITH (L Arm): Get rank "10"
WH10M-SILKY (R Arm): Get rank "5"
CR H05XS-EYE3 (Head): Get rank "1"