Unlike the aforementioned characters, the transformation of Glee's Quinn Fabray was all sorts of bizarre. She began the series in a classic 'mean cheerleader gets pregnant' trope, with a ton of potential to grow. Instead, over the seasons, the writers can't seem to figure out Quinn's direction, and she is given some of the weirdest, wildest plotlines in the series.
She waivers between kind and cruel without explanation, gets paralyzed but only temporarily, tries to steal her baby back after placing it up for adoption, enters a goth era for no reason, and apparently has an entire backstory of being bullied in junior high, though it's only mentioned once. Her drastic personality shifts totally ruin her character.