Ruzek was recruited by Alvin. Alvin went to check out the trainees and picked Ruzek, who seemed the most arrogant and active of them all. However, this arrogance started to annoy Alvin and he told Ruzek he was going to be out of Intelligence because of the way he was acting. He apologised and brought Alvin cake and Ruzek kept his job.
The first Intelligence case was based on trying to take down a dangerous criminal involved in drug trade. However, Jay was kidnapped while undercover when he was made and the kidnappers demanded all the CI files after sending Intelligence a video of Jay being tortured. Adam immediately spoke and said it was nothing to think about, that they obviously had to give up the CI files. Antonio contradicted him and said the CI's would most likely be killed and he couldn't do that. Ruzek accused him of wanting Jay 'buried' instead, which angered Antonio, and Alvin had to break up the argument. Jay was eventually rescued with the help of Erin.
Ruzek went on an undercover mission for a month or two after Kim joined Intelligence. The exacts were not known until he came back for his job. Voight told him that he went over his head to ask for the mission and didn't even tell Voight before he left. Ruzek apologises but Voight told him that his job was now given to someone else and that he'd have to go to Patrol like everyone else. However, he was given his job back the same night after the other detective left after owing a favour to Ruzek's father, despite Ruzek insisting he didn't need to.
Ruzek met Kim's sister, Nicole Silver, in Little Bit of Light. Kim told him she was coming over for a visit and that Nicole didn't like Adam anymore. This was evident when Nicole came by the 21st and met Kim and Adam, who tried to introduce himself, to which Nicole replied, "I know, I had your engagement photo on my fridge for eight months". The others were in the room and were very surprised when she said this.
He had a tough start with Alvin, who was the one who recruited Ruzek in the first place while he was in training. Ruzek's continued arrogance irritated Alvin and he eventually gave up on Ruzek. He realised he was wrong and apologised to him by bringing him cake to his garage he was living in. Alvin forgave him and they went back to normal. Alvin and Ruzek have a close relationship, as Alvin took Ruzek under his wing straight out of training, so they have a kind of 'father-son' relationship.
In season 1, Ruzek was engaged to Nicole but him and Kim began flirting and kissed. Kim was emotional and the one who got hurt when Ruzek told her they couldn't take things further. Trudy told her to get over it and that he wasn't worth her time. However, Ruzek broke off his engagement and got together with Kim afterwards.
In Shouldn't Have Been Alone, Roman's ex and old partner joined him and Kim for a case. However, their arguing during a call left Kim to start checking out the call by herself, which resulted in her getting shot in the shoulder and neck by an automated gun behind the door. She was unconscious and Adam was extremely worried, blaming Roman for the shooting. He stayed by her bed in the hospital all night until she woke up.
In What Do You Do, Burgess and Roman were taken hostage by gang members after Kim insisted on checking out a site without calling it in. However, Roman was pulled in and Kim was left out. She found a way in and found Roman bleeding on the floor and his attacker also on the ground. She tried helping him but the attacker got back up and attacked them both but she managed to take him down, two other assailants and made her way up the stairs with him. However, they realised that Kim would have to leave through the window to get back-up while Roman stayed behind. She didn't end up doing it and left Roman as bait while she waited for the last attacker to follow them up. When he did, she attacked him as well and took him down and he dies. She then goes back down with Roman and Intelligence arrives with back-up to save them both. Kim broke down in the locker room in Adam's arms after the realisation of how close she was to death came to her.
They broke up in season 3 after Roman encouraged Kim to give Ruzek the 'push test' after the two find out that Ruzek had two other ex-fiances and that Kim might be the third 'ex'. If Kim asked Adam to extend the wedding date and he agreed, then he didn't really want to get married in the first place. She tries it on him and he agrees to push the date. She's left upset and hurt and they discuss it over and break things off. Kim then joins Intelligence in season 4 after Roman and Tay leave. She comes back and Ruzek, uncomfortable with the idea of working alongside her, takes on a sudden undercover mission. Atwater tells Kim it was because he didn't want to work with her. He then returns but Voight is angered that he left without telling anyone about it and assigns him to Patrol. He is upset and comes to see Kim again, telling her that he wants to come back and be with her, to which she instantly agrees, so they get back together and Ruzek joins Intelligence again.