Gadguy
A character who’s so unpredictable, you might as well flip a coin to see what they’ll do next, and that coin might just be the same one they use to decide their fate.
'He was my guy until he flipped a coin and said, “I hate you.”'
'I trusted him until he flipped a coin and ran off with my mom.'
'He looked like a good guy until he flipped a coin and decided to betray me.'