Space Core wasn't planned to be a villian,and that's one of the 2 reasons he's the derpy dragon;the 2nd reason is he's a fan favorite despite only appearing in 2 parts of the game
The 3 robots in portal who are fan-favorites; Wheatley,who shows how moronic he is while trying to not be a moron; space core,who appears in the end cutscene; GLaDOS,who has the most screen time across both games