Rebecca and Greg first met each other at Home Base not long after she moved to West Covina, California. Greg was smitten but at the time her attention was focused only on getting back together with her ex-boyfriend Josh Chan. Despite knowing this, Greg pursued her anyway which led to a series of humiliating circumstances for him. It was only after he finally decided to give up chasing after Rebecca that she started to express an interest in him. They briefly began a relationship only for it to end just as quickly as it started. Despite Greg declaring his love for her he left town to attend Emory Unviersity. Two years later he returned and an awkward reunion ensued further complicated by the fact Rebecca had during that time a one night stand with Greg's father Marco. Their first appearance together was in the Season One episode "Josh Just Happens to Live Here!".
This isn't going to be like a roll in the hay and you go home. This is going to be three days of you and me just ruining each other. And not emotionally. Is that what you want?
When do we start?
–Greg and Rebecca Bunch, "Josh's Sister is Getting Married!"
Greg and Rebecca first met at Home Base not long after she moved to town and they went to a house party together. She made out with him at the event while looking for Josh but Greg stopped what they were doing after she started crying. He later escorted her to the Spider's nightclub which again ended in disaster. Greg would attend her housewarming party and later officially asked her out on a date. It started well but ended badly after she abandoned him to sleep with another guy. He accused her of being a bad person and after going too far to prove him wrong she apologized for her actions. After they both had a bad Thanksgiving they wound up at Rebecca's place and watched TV together. When Greg's crew went on a beach trip Rebecca got invited and she rented a party bus to transport them where she found out he was dating her neighbor Heather Davis. Greg tried to commit himself to Heather but found himself still drawn to Rebecca. Because of this he was dumped and at the same time Rebecca had an epiphany about her poor choices. She decided to begin a relationship with Greg but due to thier history he insisted they keep things casual. Rebecca eventually confessed she wanted more and had begun to truly care for him. She was disappointed when a drunk Greg's insecurities caused him to hold back his own feelings on the matter. While by himself nursing a hangover Greg declared he loved Rebecca but as he feared she had already moved on to start something with Josh Chan.
At Home Base, Greg met Rebecca Bunch who had just moved to West Covina from New York. She claimed she was there to meet Josh Chan and accepted Greg's invitation to go with him to a house party Josh was to attend. At the party Greg was put off by Rebecca's obsession with Josh. She made out with him as a distraction while she searched the party for her ex-boyfriend. When she inadvertently learned from Greg that Josh had a girlfriend she had an emotional breakdown. Greg decided to take her home and on thier way out they bumped into Rebecca's co-worker Paula Proctor. The left a confused Greg behind as Paula confronted Rebecca over her true reasons for moving to West Covina ("Josh Just Happens to Live Here!").
Rebecca failed to get into a nightclub called Spider's when she learned Josh was going there. Afterwards she bumped into Greg while at Country Market and tried to avoid him only to run into Josh and his girlfriend Valencia Perez. Josh lied about how he and Rebecca knew each when they were only grade schoolers. Afterwards, Josh found out Greg had a thing for Rebecca and encouraged his friend to go for it. Rebecca and Valencia actually became friends and Greg tagged along when they and Josh went to Spider's. A drunk Rebecca kissed Valencia and then accidentally revealed the truth about her and Josh's teenage summer camp fling. When an angry Valencia left with Josh in tow, Greg tried to comfort Rebecca but she didn't rejected his sympathy ("Josh's Girlfriend is Really Cool!").
At Paula's suggestion, Rebecca threw a housewarming party hoping Josh would attend and she'd get some alone time with him. She invited Greg who declined pointing out how horrible thier last few outings together went. Although it started off rocky, thanks to Pauala's recruiting efforts the party was a success. Meanwhile, Greg was working at Home Base and noticed his usual clientele of Gamblers anonymous recovering addicts weren't there. As he suspected Greg found all his customers at Rebecca's party but begrudgingly complimented her for throwing a good party. Greg later got drunk and danced with a giant fish. After the party was over Rebecca let Greg keep a bottle of whiskey and called him an Uber to get him home ("I Hope Josh Comes to My Party!").
Depressed about her mothers latest criticisms of her, Rebecca decided to distract herself by having a one night stand. She found she couldn't go through with it and decided to focus on making healthier life choices instead. Her declaration prompted Greg to ask her out on an official date (♫ Settle For Me ♫). She accepted after some advice from Josh and actually found herself having a good time. However, anxiety set in about starting something serious with Greg (♫ Settle For Me (reprise) ♫). She abandoned thier date to sleep with another guy only to have Greg confronted her afterwards. He was appalled by her actions and confused by her excuse that she was too screwed up to waste time on. Greg said all he could see was how the night could have gone before she ruined it and stormed off ("I'm Going on a Date with Josh's Friend!").
Sometime later Greg ran into both Rebecca and Josh Chan at Cup of boba. He told her she was a terrible person and she became obsessed with proving him wrong. Rebecca got her boss Darryl temporary full custody of his daughter but it didn't sway Greg's opinion of her. She even tried to change his mind by offering to sleep with him which Greg rejected. This only spurred her to go further in her attempts to regain his forgiveness and tried to commit a felony to win Darryl permanent custody. After being caught red handed she finally realized she'd gone too far and the matter of Darryl's custody was resolved amicably. Greg witnessed this and had a talk with Rebecca about it. He told her he still wasn't sure if she was a good person but she was at least a good friend to Darryl ("Josh and I Are Good People!").
Before Thankgiving, Rebecca was at Home Base talking to Greg and Chris. Greg revealed he was quitting his job since he had saved up enough money for business college and to move out of his father's house. Rebecca offered to buy him a drink before he left and Greg accepted. The next day, Rebecca went to Josh's family's Thanksgiving dinner hoping to score some points with him. She wound up getting diarrhea and learned Josh was moving in with Valencia. Meanwhile, Greg's fathers emphysema worsened and he was forced to use his savings to pay for expensive medical treatments. Afterwards, Greg went to Rebecca's place for that drink promised him. As they watched TV together on the couch, Rebecca suggested Greg go to night school. Greg thanked Rebecca for the advice before engaging in a playful pillow fight with her ("My First Thanksgiving With Josh!").
Over the Winter holidays they started seeing less of each other since Greg had hooked up with Rebecca's neighbor Heather Davis. They didn't reconnect until Rebecca was invited by Valencia to a go on a beach trip with her, Josh, Greg and the rest of the crew. She learned about Greg and Heather which resulted in an awkward moment. Rebecca decided to do a pole dancing routine (♫ Your Love Is My Sugar ♫) and her tryst with Greg at Bean's party was brought up. This caused conflict between Josh and Greg along with the (partial) truth of Rebecca's reasons for moving to West Covina to be known as well. Greg was confused the revelations and focused on just enjoying the beach trip with Heather. Meanwhile, Rebecca and Josh had a moment (♫ West Covina (second reprise) ♫)("I'm Going to the Beach with Josh and His Friends!").
Afterwards, Greg avoided Rebecca to focus on his relationship with Heather which after a brief rough patch ("I'm Back at Camp with Josh!") became more serious. Greg reprimanded himself when got pulled back into Rebecca's orbit after another scheme she had went poorly again ("That Text Was Not Meant for Josh!"). Greg warned Josh of Rebecca's true intentions but he didn't listen and got tricked into joining a class action lawsuit she put together. Greg teamed up with Valencia to expose Rebecca lies only for their plan to backfire ("Josh and I Work on a Case!"). Greg got Heather a job at Home Base only to lose focus again when he saw Rebecca on television trying her case. Heather realized he still had feelings for Rebecca and she broke up with him (♫ Don't Settle For Me ♫). She also encouraged him to go after Rebecca but when he did Greg found Rebecca kissing Josh ("Josh and I Go to Los Angeles!").
So you were going to leave without even saying goodbye to me?
I couldn't say goodbye to you. If I did I'd never leave.
–Greg and Rebecca Bunch, "When Will Josh and his Friend Leave Me Alone?"
After Rebecca slept with Josh they began a friends-with-benefits arrangement and he moved into her apartment. At the same time Greg got caught driving drunk, was sentenced to rehab and went into seclusion for a month to deal with his alcoholism. Rebecca learned about what happened to him and gave him space to deal with his issues. When they met again he found out what happened with Josh but accepted that he blew it and told Rebecca she deserved someone who made her happy. Things between her and Josh didn't last long as a pregnancy scare resulted in him dumping her. Depressed about the break up she went to the park and had a chance encounter with Greg. They wound up kissing and Rebecca suggested that they could start seeing each other again with a clean slate. They planned to meet later but Greg had second thoughts and stood her up. He decided to seize an opportunity his father gave him to finally attend Emory University. After Greg said his goodbyes to his friends at the airport he was surprised when Rebecca showed up. Greg told her he loved her and would never forget thier time together. Rebecca watched tearfully as he ascended on an escalator to board his flight.
In the midst of a downward spiral Rebecca Bunch wandered into a local bar that Greg Serrano used to frequent. She mentioned this to the bartender who remembered Greg used to study at the bar and would mention frequently about how he was accepted to Emory. Rebecca talked to the bartender about how hopeless and confused she was feeling when she got an unexpected phone call from Greg. Rebecca was ecstatic as she related to the bartender how Greg was always there for her when she needed him. She was crushed when it turned out the call was just an accidental butt-dial overhearing Greg in the background ordering a burger. In a coincidence Greg's father Marco is also at the bar and a reluctant Rebecca goes over to say hello. She learns from Marco that Greg is doing well at Emory and is now engaged to a fellow student there. This further depressed Rebecca who tried to pry more information about this woman from Marco. He refused to answer as he didn't want her to get in the way of his sons happiness. He did try to cheer Rebecca up by complimenting her which led to the two of them having sex. In an act of contrition, Rebecca would later confessed this happened to Nathaniel Plimpton III, Josh and Paula.