Okay this is a very unfair question because the majority of the female characters have died tragically.
As much as I truly want to choose all of them, I first want to talk about Abaddon’s death which I felt to be quite sexist. Her death sadly followed in the footsteps of Eve’s death; it just ruined her in the sense that all of her established power and intelligence was completely thrown out of the window simply for the Demon Dean story arc. As someone who looks up to strong, authorative women, I really didn’t think the show would be so misogynistic as to kill her character off. I was wrong.
The vampire Lenore, who appeared in 2.03 “Bloodlust” and 6.19 “Mommy Dearest”, I also thought had a lot more to offer and, similar to Tessa’s death in 9.22 “Stairway to Heaven” they could have killed off any character. It was unnecessary to bring Lenore back into the show for 5 minutes just to have Castiel kill her to assert his authority. I also really liked April, a Reaper like Tessa, who appeared in 9.03 “I’m No Angel”. She could have had a lot of potential but I understand that she was a clear and obvious villain. Tessa, however, was not a clear villain and yet was still murdered by Dean, mirroring what happened with Abaddon, to basically further develop Dean’s dark character.
Despite all of these unworthy deaths, the character I wish was still alive would most definitely be the Angel Muriel who appeared in just one episode, 9.09 “Holy Terror”. Even though her screen time was very minimal, she made a big impact on me. Muriel was a female character with personality and moral. She makes her entrance through hearing Castiel’s prayers and comes to help him which shows her kindness and compassion.
When she gets to Cas she realises who he is; a traitor and enemy of Heaven who almost every Angel is out to kill. She knows that if she helps him that it could put her life in danger (and it does) but she still gives him information about the Angel war nonetheless. Muriel explains how she wants to stay neutral in the war between the Angels which is why Castiel states “we’re similar”, as they both want the fighting to stop. Due to this quick bond between them, Castiel tries to convince Malachi that she is innocent, when they both get captured and tortured by him. Unfortunately Malachi gives the order to kill Muriel because of her interaction with Cas. (I’m surprised there’s any Angel’s left at this rate.)
Personally, I believe that Castiel and Muriel could have had a very close, strong friendship or maybe even a romantic relationship. I don’t think that it was necessary to kill Muriel in this episode as the severity of the situation is already established in the opening scene where Angels are seen brutally slaughtering other Angels. Furthermore, Castiel escapes and murders his way out of the place where Malachi has him prisoner and this alone indicates the war and battle atmosphere. I don’t think that Muriel had to die for the story or the mood of the episode to be made clear. Castiel and Muriel could have both made it out alive and stayed in contact with each other and she could have easily been one of the many followers that Castiel gathers later on in Season 9. She could have replaced the useless, annoying Hannah in being Castiel’s right-hand lady. I feel that Castiel and Muriel would have had a much more respected and close friendship than the untrusting and unstable relationship that Cas and Hannah have.