Not much information has been disclosed about Beth's life prior to "The Bittersweet Science", other than she had a father who would always say "Go big or go home". Her father passed away just a few months prior to her meeting Hotch, but she and her mother had moved on from the tragedy.
Beth first encounters Hotch while both are training for their respective triathlons, coincidentally on the same route. When they stop at a park, Beth talks to Hotch and both realize that one another is participating in a triathlon. Having known about Hotch's biking skills beforehand, Beth asks him for some tips on how to ride one, as she isn't the best cyclist. Hotch agrees to let her know when he is available and she gives him a card before telling him she knew he was an FBI agent after seeing his suit. She then leaves.
After he returns from a case, Hotch meets up with Beth at the same park, where he learns about her father. Both smile at one another before Beth bikes off, with Hotch in pursuit.
Hotch and Beth continue training for their triathlons, and the latter cheats while they do a practice run. Hotch jokingly states that her cheating was "pathetic" before deciding to ask her out on a date, which she accepts. When Hotch returns after solving a case, he goes to Beth's house, where she greets him. Beth then kisses him on the lips before the two head out, signifying the start of a romantic relationship between the two.
Beth has a date with Hotch in a café, with Hotch buying her tea. Hotch tells her that since he might be leaving for a case and he might miss his race. However, he says that there's always other races and that he might even race in hers. Beth then mentions Jack being at his race and Hotch tells her he'd like the two to meet. Beth later watches Hotch finish the race and meets Jack. Both take a liking to one another, and Hotch escorts them away.
Beth visits Hotch, bringing with her breakfast, and happily greets Jack before he shows her a bed he and Hotch had made. The three then debate on whether to sleep in this bed the following night or at Beth's. Beth later babysits Jack while Hotch attends to negotiating a hostage situation at a bank. At the end of Run, Beth attends JJ and Will's wedding, having an awkward first meeting with Garcia. She is later shown dancing with Hotch.
Beth tells Hotch that she got a job offer in New York. He encourages her to go there since that is a great opportunity for her but she hesitates because that is also about their relationship. Hotch does not mean to break up with her but to consider what is best for her and he suggests a long-distance relationship and to visit her when having the chance to.
After spending some time away from Hotch, Beth is able to have a visit from him and Jack as they visit New York. The two comment on how the long-distance relationship was working out, just before Hotch receives a call from Sean, his brother, who witnessed a death occur in the bar he worked at. Beth persuades Hotch to help Sean, citing that they were brothers and that their relationship was more important. She is left to take care of Jack and give him a tour of the city. By the end of the episode, Beth and Jack meet Sean.
Though Beth isn't physically seen in the episode, Rossi stated that she was on a business trip to Italy. She is later seen several times in the vision of a video montage of Hotch's recent life, which is viewed by him and also visions of Haley Brooks and George Foyet (Hotch's deceased ex-wife and arch-nemesis, respectively), who both approve of her, with Haley saying that Jack really likes her. Later on, Haley tells Hotch that Beth is "a keeper" and that he mustn't "screw up" before leaving. By the end of the episode, Hotch calls Beth.
Beth was mentioned throughout the episode, where it is revealed that she and Hotch have decided to end their relationship after the latter encouraged her to take a job position in Hong Kong.
- Coincidentally, in The Bittersweet Science, Beth's debut episode, there is a character named Dr. Jean Bellamy portrayed by Sean Kathryn O'Connor. The character's surname seems to be a reference to Bellamy Young, the actress who portrays Beth.
- In Amelia Porter, the character of Beth was retired from the show due to Bellamy Young's commitment to another show she was starring in, Scandal.