Loki

Loki, the God of Mischief, knows how to make life difficult for Thor and anyone else he views as a threat. At one point Loki used his shapeshifting abilities to turn himself into Sif, Thor’s ex-lover. This switch also accompanied some time traveling, which Loki used to kill his grandfather Bor. Loki also convinced Asgard that their ally, Beta Ray Bill, was a villainous Skrull. Thor caught on eventually, but by then, the damage was done.