In Halo 3: ODST for the Xbox 360, there are a few bonus characters that you can unlock to use in Firefight missions.
Buck – Complete Tayari Plaza on normal difficulty or higher
Dare – Complete the campaign on legendary
Dutch – Complete Uplift Reserve on normal difficulty or higher
Mickey – Complete Kizingo Boulevard on normal difficulty or higher
Romeo – Complete NMPD HQ on normal difficulty or higher
