Initially, Holly replaced Toby Flenderson as Human Resources Representative at Dunder Mifflin Scranton when Toby left for Costa Rica. Her character was later developed into a friend and romantic interest for Michael Scott. She and Michael have a shared sense of humor and similar personality traits, though Holly does prove to be more rational than Michael. In the office, she is noticeably more acquiescent to Michael's antics and ideas than was her predecessor, Toby. She and Michael eventually marry, move to Colorado, and start a family together.
Holly is originally from Des Moines, Iowa, and her parents now live in Boulder, Colorado and suffer from dementia. In her early life, she held the position of hall monitor in middle school due to her geeky demeanor, and because of this, was a target of harassment by other students. When she was in high school, she was diagnosed with mono.
Holly arrives at Dunder Mifflin to serve as Toby's replacement. Michael initially dislikes her because she is part of Human Resources, but eventually falls in love with her. She is subjected to hazing by Dwight Schrute, who convinces her that Kevin Malone is mentally challenged, a belief she continued to harbor until Season Five when it was embarrassingly pointed out to her that he wasn't. During an attempt to put a raccoon in Holly's car, Dwight is berated by Michael, who takes Holly's side and loudly proclaims that she is the "best thing that has happened to him and the company since World War II." Holly invites Michael to go out for dessert after Toby's going away party, but Michael turns her down, opting instead to go with Jan Levinson, who is pregnant and had asked Michael to be there as if he were the child's father.
Michael had been courting Holly in the first episode of the season. Holly practices yoga, so Oscar refers Holly to a yoga class with an attractive teacher. Holly agrees to a date with him, much to Michael's dismay. Holly buys tickets for Counting Crows, but the yoga instructor doesn't call her. Michael, who does not pick up on several hints that Holly has given him (about attending the concert with her), purchases the tickets from her and rips them to pieces. She ultimately considers him to be her best friend in the office. It is also revealed that she was picked on as a child just like Michael. Michael is pleased when Corporate HR blasts Holly for seeking disciplinary action against Meredith over the sex-for-supplies (and Outback Steakhouse coupons) deal instead of conducting a stupid seminar and getting signatures, but he also yells at the staff to complete the seminar. This also restores good feelings between Holly and Michael.
Michael tells Holly he will treat her poorly in front of Jan during her baby shower because he has a vested interest in Jan's baby, which is not his. Despite being told of this, she is shown to be uncomfortable with Michael's attitude toward her. However, despite witnessing Michael's treatment of Holly throughout the day, Jan notices the two of them have a lot in common after Holly makes jokes in front of her. Jan then orders Michael not to date Holly before driving away. This, in addition to feeling no connection with Jan's baby, prompts Michael to apologetically hug Holly and ask her out on a date, which she accepts, visibly moved.
Holly and Michael date, and on their third date have sex in the office (inadvertently leading to the break-in at the office). She helps Michael organize a fundraiser (offering yoga lessons) and displeases CFO David Wallace, who sees them kissing, thus noticing a relationship. In response, she is transferred back to Nashua. She and Michael plan to continue dating, but realize on the trip that the distance is insurmountable. Thus, they are forced to end their relationship, sharing one last embrace before saying goodbye. Michael tells everyone that they are engaged upon his return, but later admits this is not true. Later, David is called out on the transfer by Michael, who chastises him on the phone before hanging up.
When Michael comes to Nashua with Pam Beesly to give his lecture and find closure with Holly, she is not there because she is on an HR retreat. It is also revealed that she is dating one of the salesmen there, A.J., which deeply upsets Michael and makes him unable to do his presentation; he ultimately breaks down and asks her boyfriend about Holly in front of everyone before leaving. He goes to her desk and smells her sweater, cutting a sleeve off to take with him. Michael finds a file on her computer titled "Dear Michael" and copies it to his flash drive. He tells Pam about it, and Pam volunteers to read it herself and deletes it afterward, telling Michael it said she still has feelings for him and that it isn't over. It is unclear whether Pam was telling the truth.
Holly and Michael reunite at the Dunder Mifflin company picnic for the first time since she was transferred. She is still seeing A.J., who arrives with her. Holly and A.J. have apparently started living together. David Wallace lets Holly and Michael do a skit about the history of Dunder Mifflin; the two decide to do a skit that spoofsSlumdog Millionaire. The presentation goes horribly and gets worse when they reveal that the Buffalo branch is closing down, much to everyone's surprise. Michael and Holly reminisce about the presentation and talk about one in the future. Holly leaves with A.J., with Michael noting that it wasn't the right time to talk to her and that they will eventually find each other.
Holly does not appear in Season 6, but is mentioned many times by Michael, who still has feelings for her. Her return is hinted at in the season finale, "Whistleblower," after Michael takes the fall for Sabre's defective printers. In return for his loyalty, Michael requests that Holly be returned to Scranton. Jo says she will see what she can do.
Holly first appears over speakerphone in "Sex Ed" when Michael calls her, fearing he has herpes and may have given it to Holly. Their lighthearted conversation turns serious when Holly suggests that Michael over-romanticized their relationship, which she characterizes as "just a fling."
In "Classy Christmas", Holly returns to Scranton to fill in while Toby is away at jury duty. The initial excitement about seeing her old friend again turns to anger when a jealous Michael vandalizes her Woody doll (a gift from AJ). Holly divulges to the women of the office that AJ won't commit, and she's going to give him an ultimatum to propose to her by New Year's. When Pam tells Michael, he is cheered up by the news, as well as Holly lying to AJ about how Woody got damaged.
Holly returns from the holiday break in "Ultimatum" without an engagement ring. Michael is briefly overjoyed until he learns that Holly gave up on her ultimatum, considering it unreasonable. However, at the end of the episode, Holly tells A.J. that she wants to take a "break" from their relationship. Her and Michael's friendship seems to rebuild itself with "The Seminar".
In "The Search", Holly rejects Michael's advances, saying she doesn't want to be involved with a coworker again. When he goes missing, Holly is able to track Michael's footsteps due to their similar thought patterns. When she finds him on a building rooftop, they admit that they miss each other and share a kiss.
In the episode "PDA", it is revealed that Michael and Holly are officially dating. They make the rest of the office uncomfortable with their excessive public displays of affection, and agree to stop only at the insistence of their coworkers. Later that day, when Michael declares that he loves Holly and she says that she returns his feelings, they must settle for a handshake. In order to avoid the kind of breakup they have previously experienced, they decide to move in together.
As the season progresses, Holly's presence leads Michael to become a more mature adult. In "Threat Level Midnight", Michael screens the film that he had been working on for the past 11 years. Michael is hurt when Holly isn't impressed, but her honesty helps him realize that the film is laughably awful. In "Todd Packer", Todd Packer returns to the sales bullpen. She initially trusts Michael's assessment of Packer, but confronts Michael about his friend after Packer shows his true colors. When Michael tries to smooth things over, Packer speaks disrespectfully about Holly. Michael decides not to disclose that Packer's "promotion" in Florida is a prank by Jim and Dwight, and as Michael and Holly watch him drive away it's obvious that she is helping Michael to grow. A deleted scene from this episode reveals that Holly has a blind friend named Lois, who doesn't get along with Michael.
In "Garage Sale", she realizes her aging parents need assistance and decides to move home to Colorado. She briefly changes her mind when Michael seems reluctant, saying that he is her life now. At the end of the episode, Michael leads her through various spots in the building that are meaningful to them (the stairwell where they first kissed, the water cooler he visited just to watch her work). He finally leads her to the annex which is filled with lighted candles. He begins to propose to her when the sprinkler system goes off. Through her laughter, she accepts his proposal. Afterwards, he announces to the office he's quitting his job to move to Colorado with his fiancée.
Holly does not appear in the subsequent episodes "Training Day" and "Michael's Last Dundies", but did have a voice-only segment (in a telephone conversation) in "Goodbye, Michael", confirming with Michael that she'll pick him up at the airport when his plane lands. In the process, she unintentionally cheers a distraught Michael up when he initially gets cold feet about moving to Colorado. The episode marked Steve Carell's final episode as a series regular.
Holly does not appear during season nine, but Michael makes a surprise appearance at Dwight and Angela's wedding in the series finale. During the wedding reception, Michael shows Pam pictures of Holly and Michael's kids on his cell phone. Pam reveals that Michael and Holly are living very happily together. Michael now has two cell phones to store all the pictures that he takes of their kids, and is very happy to finally have a family cell phone plan. NBC.com later revealed that Michael and Holly had four children together.