Karen Singer was Bobby Singer's wife, and her death was the reason why Bobby went into hunting.
Not much is known about her other than the fact that she was married to Bobby Singer. Three days before she becomes possessed by a demon she and Bobby had a fight. Karen wanted kids, but gets extremely angry when Bobby tells her that he breaks everything he touches. With the demon inside her, it caused her to be rabid and Bobby inadvertently killed her as he didn't know about demons beforehand. Eventually Rufus Turner exorcised her, but Karen died of her wounds, leaving Bobby with great guilt. After she died, Bobby cremated her body. Her death pushed Bobby into the hunter lifestyle.
Karen appears in Bobby's nightmare, very angry at Bobby for killing her. She chases Bobby all over the house in the state she was probably in when she died. She vanishes when Bobby realizes it's a dream.
The Horsemen Death resurrects Karen to send Bobby a message. So she returns to Bobby, who is shocked to see her. She is unknowingly tested by Bobby. When Sam and Dean catch on people returning from the dead, Bobby protects her. She meets the boys for the first time when Bobby brings them home. She offers them pie, which she had finished baking.
Later on she catches Dean watching the house and invites him in. Once inside she mentions to Dean she can't stop baking and hasn't slept since she came back. Plus she remembers Bobby killing her while being possessed, but she hasn't told Bobby in order to protect him. Then she collapses and Bobby brings her to bed. She knows she is beginning to turn and tells Bobby she remembers. She asks Bobby to kill her and gives him Death's message. Bobby, once again, is forced to kill her, and burns her body again, but away from the other zombies.
When Bobby lies in a coma, his subconscious surmises that a possible way to recover is to access his darkest memories. One of them occurred a few days before Karen was possessed by a demon and he was forced to kill her. She wants to talk to Bobby, and during the conversation he reveals that he doesn't want to have children, something he has never shared with her. Karen is extremely distraught. Bobby reflects that what was worse than killing her, was knowing that they never got past this argument.