A Helljumper is an ODST[DF](Orbital Drop System Troopers [Defensive Forces]).I might be wrong about the ST part but the DF part can be seen on Halo 1's Johnson's hat or on varios places on Cairo Station.
I used to own a petting zoo until the marines found a radio and called for backup.It was a sad day for Joe, our main grunt, and Sammy the E lite.