You don’t know what “Default Pawn” could be? the name is self explanatory, if you don’t know what it is then something tells me you don’t know what Pawn is 
Pawn is a lowest class of animated controllable actor that can be possessed by controller. Character class is also Pawn, difference is it has some extra code helps control humanoid/skeletal pawns. Look in “Class Viewer”
GameMode class has default spawn system, which by default (if you didn’t override anything) spawns “Default Pawn” to random PlayerStart on level and automaticlly possess the pawn to specific player controller. I think insted of using “Character” which is base class, if you want to disable default spaning system you need to set it to “None”, then nothing will be spawned.