His romantic relationship with Levi Schmitt began when Nico winked at Levi while operating, confusing Levi. ("With a Wonder and a Wild Desire")

Later, at Joe's bar, Nico bought him a beer after a rough day. Levi wanted to pay him back and grabbed his wallet, but Nico touched his hand to stop him and said Levi could buy the next one. Nico then left, with Levi staring after him. ("Broken Together")

Nico later came into the residents' locker room to shower despite having access to the fellows' lounge and Levi was shocked and distracted by seeing Nico with no shirt. ("Gut Feeling")

Later, Nico then asked Levi if he would be willing to get that drink that evening, but Levi said he might have plans later. Nico just said he'd be at Joe's if his thing got canceled. Later, Levi came to the bar for that drink, but Nico mostly brushed him off, confusing Levi. ("Everyday Angel")

After working together on J.J. Williams, Levi and Nico ended up in an elevator together and they kissed. They stopped when someone else got on the elevator, but once they were alone again, Nico apologized for what he'd done. Levi said it was fine and also that it was his first time and he was happy to learn. However, Nico said he didn't want to teach him since he had already dealt with his coming out and didn't want to go through it again. ("Flowers Grow Out of My Grave")

After being trapped together in an ambulance during the windstorm, Levi talked about having feelings for men before, but never exploring it because he was a nerd and never considered sex much at all. After his confession, Nico kissed him and the two of them had sex in the ambulance. ("Blowin' in the Wind") They have been in a relationship since.

When Levi's mother called while he was hooking up with Nico, Nico was upset when Levi shushed him. Levi tried to explain why his mother didn't know about Nico, but Nico wasn't persuaded and left. Later that day, Levi explained to Nico that his mother likes to talk about everything as if it's going to hurt him and he wants to lock Nico in his own eyes before inviting his mother's criticisms because Nico's his first love. Hearing that Levi loved him, Nico kissed him and they started to walk away. Levi asked if he's not going to say it back. He said he would, but he wanted to make Levi sweat for a minute first. ("Blood and Water")

Levi learned that Nico had an upcoming job interview with a hospital in San Francisco, and expressed worry about a long-distance relationship through bringing up sports teams. While together on a case in which Nico had to fill the man's body with cement, Levi continued complaining about Nico's proposed move. Later, it was revealed that Nico made a mistake during the surgery, and the patient collapsed. Bailey ordered Levi to fix the man, as he was the grandson of a friend of hers, but things didn't go well, and the patient died. Later, Nico was about to do a Skype interview with his new prospective boss, and Levi attempted to persuade him to leave and re-schedule the interview. Nico spoke sharply to Levi, saying that he was not a failure, and implied that Levi is, which broke his heart. Levi was angered when Nico started the interview and slammed the door behind him. ("Head Over High Heels")

Nico was annoyed that he is relegated to simple tasks in the wake of his surgical error. Levi, on a high due to him saving the Seattle Fire Chief, ran to tell Nico about it. However, Nico didn't want to hear it and was rude to Levi, and demanded Levi's help in fixing a shoulder dislocation. Later, Link told Levi to leave Nico alone, and that if he wanted to talk, he should wait for him to say so. ("What I Did for Love")

Nico later realized via Link that his past behavior had been unacceptable, in the wake of a fog throughout Seattle, which led to a massive car pile-up on the freeway. He tracked Levi down, only for Levi to give him the cold shoulder, which immediately turned into lecturing on Levi's end when Levi informed him that he missed him. Levi told Nico about his day, and Nico listened, which culminated in Levi informing Nico that he loved him, but he still deserved better than how Nico had treated him. Nico came clean about his anger due to his failure, and that he was jealous of other doctors in the hospital, Levi especially, due to their ability to bond with patients and have successful moments. Levi immediately told Nico that he could help him, and wanted to. Later, the two tearfully watched My Girl in Levi's basement, and his mother came downstairs with chicken noodle soup. She was very welcoming towards Nico, but called him Levi's "friend". Levi immediately informed his mother that Nico was his boyfriend that Nico was the one who had been having a bad day. Mrs. Schmitt then offered up the chicken soup to Nico, after he informed her that he liked it, telling Levi that they would talk later. Nico told Levi that he was "amazing," and Levi put his head on Nico's shoulder. ("Jump Into the Fog")

After the other residents learned that Levi had been the one to turn Meredith in, they started ignoring him, which upset him. He didn't want to go to work because of it, so Nico promised him a reward after he got through the day. At the end of the day, they were having a drink at Joe's and Nico said he could have his reward after they got some food. However, this was interrupted when a car crashed into the bar. ("Let's All Go to the Bar")

Nico chafed when Levi started staying with him after moving out of his mother's place while looking for a place of his own. Nico didn't feel they were ready to cohabitate and told Levi that. Later, after seeing Irene Sholman dance with her husband, Levi told Nico he was looking for his dance partner. ("Save the Last Dance for Me")

Their relationship was further strained when Nico didn't want Levi to meet his parents, even though they were in town. Levi said he could take the criticism, but Nico admitted he'd never told his parents he was gay, despite having told Levi he didn't want to live in Levi's shame spiral. ("A Diagnosis")

Their relationship continued to struggle, with them only having sex and not talking about the situation they were in. Levi expressed his desire to have a relationship that was more than just sex, but Nico blew up at him, saying that he didn't want to talk about his feelings and he didn't understand why Levi couldn't just accept that. He said if Levi wasn't okay with that, they should end things. Levi said no, he could be okay with it and they proceeded to have sex. ("Snowblind")

Later, when Link decided to recommend Nico for the job with the Mariners, which Link himself was turning down, Levi decided it was time to end things between them, as Levi felt he was always the one making sacrifices for their relationship. ("Give a Little Bit")

After the start of the pandemic, Nico went to Levi. He apologized for not being a good boyfriend, but offered to provide Levi with help de-stressing, starting right away in the supply closet. ("The Center Won't Hold")

Nico's arrangement with Levi developed to the point where they had essentially resumed their relationship, to the point where the pair were comfortable joking about their history. After some weeks, Levi finally told Nico he was upset that Nico was unwilling to give any space in his apartment to Levi, even a drawer. In response, Nico told Levi he loved him and asked him to move in. In a complete reverse of their feelings months earlier, Levi made an excuse to leave the conversation without answering. ("Good as Hell")

After this conversation, Levi avoided Nico for several weeks. Nico confronted Levi, asking if they were breaking up, if he wanted to see other people. Levi said he didn't. After the doctor who administered his vaccine for the vaccine trial flirted with him, Levi went to Nico's place, where he learned that Nico had been lighting candles every night in the hopes that one of those nights, Levi would show up. He confessed his love to Levi and they kissed. ("I'm Still Standing")

After Levi did two laparascopic cholecystectomies in one day, he and Nico had sex in a supply closet. ("With a Little Help From My Friends")

Levi ended things between them after losing a patient in surgery, also telling Nico that he wasn't going back to work. ("Living In a House Divided")