Gray is a psychopathic and brutal killer and Cream is a sad rabbit who died after running into a wall of spikes.
Gray is being manipulated by Black and could never go against him. Gray is also insane and the adopted son of Black.
Cream is one of Sonic.EXE's puppets and got her soul taken by Sonic.EXE. Cream is also shy and gets scared easily. 
Gray was once a normal crewmate but Black ejected his dad. Cream was once an innocent rabbit but she ran into a wall of spikes and died.