I wasn’t absolutely sure if this meant a character who the writers didn’t write very well (undeveloped, overshadowed etc) or a character who the writers make go through the most sh*t. I’m going to assume it means the latter and just say that most of the female characters get screwed over.
For obvious reasons, Jessica Moore is a contender in this category after being brutally murdered just to set the plot and then disrespectfully impersonated quite a few times over the course of the show, including being impersonated by Lucifer himself.
There is also Linda Tran who goes through getting possessed, tortured, captured and imprisoned purely to keep her son safe, just to find out that he is dead when he appears as a ghost.
However, nobody gets screwed over as much as psychic friend Pamela Barnes…
Appearing in only four episodes (one of which being dead and living in Heaven), Pamela had to endure having her eyes burnt out at seeing Castiel’s true form and then being stabbed by demons whilst putting her life on the line to help Sam and Dean. When we first see her she is fun, flirty and care-free despite having strange, psychic powers that would normally cause people to feel more isolated and introverted. Then, after only her second scene, she is blinded and although she doesn’t hold a grudge and continues to help the Winchester’s with their problems, I felt that she was a bit more distant with them, deep down blaming them for what happened to her.
She keeps helping the boys regardless, having a great moral compass, and is killed in the process, which the boys blame on themselves. Although, I will applaud the writers for including Pamela in 5.16 “Dark Side of the Moon” as being content in Heaven with Ash, to which she can be at peace, and Sam and Dean can finally forgive themselves for completely ruining her life.