Tammy Guetermann is a deep-cover agent for the Galactic Federation, and the daughter of Pat Gueterman and Donna Gueterman. She was also a student at Harry Herpson High School and the close friend of Summer Smith. She became engaged to Birdperson and planned a wedding as a ploy to gather as many enemies of the Galactic Federation in one location in order to capture and arrest them. After the fall of the Federation, she became the leader of the remains, and will likely serve as one the show's major antagonists.
Tammy made her first appearance in the episode "Meeseeks and Destroy", where she made a brief cameo in the audience during Mr. Meeseek's assembly about the benefits of a friendship with Summer, and can be seen gathering around her afterward. She makes another cameo appearance in "Rick Potion No. 9," later, she became a Cronenberg. An alternate version of Tammy appeared again and had her first major role in the episode "Ricksy Business" at Rick and Summer's party. After seeing Brad arrive at the party she asks her friend to mess up her hair so she looks drunk. Tammy however starts talking to Birdperson who is apprehensive about Tammy's overt desires. When Birdperson tells Tammy that he is not looking for a soul-bond Tammy responds by whispering something in his ear to which he replies "I believe Birdperson can arrange that." Ultimately, Tammy goes home with Birdperson at the end of the party.
She later appears in the episode, "Get Schwifty" in Birdperson's home, casually dressed in a robe, implying their relationship has continued, and making a sly innuendo that Birdperson finds uncomfortable. She is later seen watching the transmission of the events back on Earth with horror.
In "The Wedding Squanchers", Tammy reveals during her wedding reception speech that she is a deep cover agent for the Galactic Federation. After declaring that everyone at the reception is under arrest she shoots a confused Birdperson several times in the torso with a laser rifle after he ignores her order to sit down. In the ensuing shoot-out she exchanges fire with Rick and is last seen falling over with Squanchy stepping on her foot.
Tammy's parents are the spouses Gueterman, Pat and Donna. They briefly engage in conversation with the spouses Smith, Jerry and Beth before the ceremony, and are later revealed to be cyborgs, likely as part of Tammy's cover. They pursue the Smith family, and immediately after boarding their getaway vehicle, they both fall off the back of it while it's in mid-air.
In "The Rickshank Rickdemption", Tammy is revealed to have survived the shoot-out in "The Wedding Squanchers" and is now the leader of its remaining party. It is also revealed that the Galactic Federation has revived Birdperson, now renamed "Phoenixperson", as requested by Tammy.
Tammy is a teenage white girl with short dark brown hair. She wears a blue sweater which exposes her belly and tan pants. In "The Wedding Squanchers", she wore a short, white wedding dress, heels, flowers, and a pinecone as her wedding ring from Birdperson. In "The Rickshank Rickdemption", she wore a bright, shiny, red and black leather suit, with an upturned collar, and silver shoulder pads with the Galactic Federation symbol on its sides.
Tammy is a polite and sarcastic girl. She is also a very insecure and worrisome girl, with a lot of social anxiety. Her biggest problem is that she doesn't have a boyfriend and she feels like she needs to act cool in front of them by doing immoral things such as drinking and watching abnormal kinds of pornography. She's very boy crazy, overall and some of the porn she watches, such as bukkake, interests that part of her. When she started dating Birdperson, she was finally happy and secure with her life, but she was still very sex-crazed around him and didn't care to keep those feelings private when other people were around.
In the episode "The Wedding Squanchers", it was revealed that her entire personality was fake, and that she was just an undercover agent from the Galactic Federation who put on a fake girl for a period of time, in order to catch Rick Sanchez, meaning almost all of this was fake. However, it's likely that the only feelings that she weren't fake was that for her husband, as she revived him as Pheonixperson. Though it could be deduced that she revived Birdperson solely for the purpose of helping her capture Rick.
- She likes watching Bukkake but she doesn't know if she would ever do it. This may not be true and just be part of her act as a high school student. 
- She goes through three different tones of voice. Her first was the voice of a regular teenage girl. After meeting Birdperson, she started talking in a seductive, sexual voice. And then after revealing herself to be a member of Galactic Federation, she talked in a very strong and angry, but professional voice.
- It is unknown if Tammy Guetermann (C-137) was working for the Galactic Federation in Dimension C-137.