I don’t know if anyone has already done this, but I was thinking about this for my own purposes so I thought I’d share. A few of these characters are clearly molded by a certain writer and others seem to have been formed by more of a group effort. I’m listing the character for the most part under the writer who wrote the first episode in which they appeared, but I’m aware that doesn’t necessarily mean that’s who had the original thought. If anyone has corrections, please let me know. (I spent a ludicrous amount of time compiling this, but I’m sure there are errors regardless.)
- Krissy Chambers - Adam wrote both 7x11 and 8x18, the two episodes in which Krissy appears. He named her after his cousin. She was inspired by the young lady character from True Grit.
- Abaddon - "People are asking how I came up with Abaddon? I knew I wanted to write a femme fatale character. I just needed a story for her and a why? So when Andrew Dabb and I started talking about Men of Letters and Henry we needed a new big bad. But Crowley was so great. Couldn’t top him. So, why not go another way. Woman. Bad ass. Something more then a demon. Research revealed Knights of Hell. And in it I saw the name Abaddon. I knew then we were on to something. I modeled her after Lauren Bacall. Wrote her that way too. Then Alaina Huffman came in and read and I almost jumped out of my seat. She was perfect. Jeremy, Bob and I all agreed instantly that she was Abaddon. And the rest is #SPN history.” (source)
- Henry Winchester - Adam wrote both 8x12 and 9x17, the episodes in which Henry appears.
Dan Loflin and Andrew Dabb
- Garth Fitzgerald - Loflin recounts Garth’s origins here. Loflin and Dabb wrote 6x4 in which Garth is first mentioned, and 7x8 in which he first appears. Adam Glass wrote Garth’s other two appearances in 7x18 and 9x12.
- Aaron Bass - Edlund wrote Aaron’s only appearance in 8x13. (I wouldn’t include Aaron since he’s only been in one episode, but people seem to want him back and he isn’t dead, so who knows? He’d be written by someone else, though, if he were to return.)
- Metatron - Edlund wrote 7x21 in which Metatron is first mentioned and 8x21 in which he first appears. His subsequent appearances were written by several different writers.
- Crowley - Edlund wrote 5x10 in which Crowley first appeared (though he was mentioned before that). He has, of course, been written by many other writers since then, but I’d credit Edlund with most of what makes Crowley, Crowley.
- Kevin and Linda Tran - Edlund wrote 7x21 in which Kevin and Linda make their first appearance. Since then both Kevin and Linda have been written by several of SPN’s writers.
- Death - Although he was mentioned several times earlier, Gamble wrote 5x21 in which he first appeared. She also co-wrote 6x11 and wrote 7x1.
- Jody Mills - Carver wrote 5x15 in which Jody first appeared but she has been written by several writers since then.
Group Effort (Basically Dean’s, Cas’, and Sam’s flashback and weird snippet characters when they were introduced. I would think someone worked out the major aspects of their characters, and I wouldn’t be surprised if the flashbacks weren’t actually written by the writers credited in the specific episodes, but that’s just a guess on my part.)
- Benny Lafitte - Carver wrote 8x1 in which Benny appears, but we learned much more about him in subsequent episodes 8x2, 8x5, 8x7, 8x9, 8x10, and 8x19 so I think this is probably more of a group effort example
- Naomi - Buckner and Ross-Leming wrote the episode in which she first appeared, 8x7, but she was written by many other writers in 8x10, 8x17, 8x19, 8x21, 8x22, and 8x23.
- Amelia Richardson - Carver wrote 8x1 in which Amelia first appears, but as with Benny, she was seen in flashbacks in episodes 8x3, 8x5, 8x6, and 8x8, before we learned more about her in current time in 8x9, and 8x10 and she was written by several different writers.
I know a few of these characters are from before the Carver era, but since they’re still around I thought I’d include them.