Joe's troubled younger brother, Leon Cruz, is in the street gang, Insane Kings, and he is desperate to keep him out of harm's way. He is haunted by a decision that he made on the job relating to his brother that later effects his own safety, and the safety of his fellow firemen. In a fire, Joe saw Flaco, the leader of his brothers gang, and left him to die.
Casey said he would go to the police if Cruz didn't resign - however, Casey later reconsidered his decision. Leon was after able to leave the gang and even went to university. In Season 2, against Cruz's will, Leon later took up undercover mission with the Intelligence Section to take down the new chief of the gang, after a little girl was killed in a drive-by shooting in his neighborhood. Leon completed the mission safely, but for his protection he was sent out of Chicago to stay with their Uncle Jorge.
At the end of Season 1 and beginning of season 2, Joe begins a relationship with Zoya, Otis's immigrant cousin, and he decides to attempt to marry her to allow her to stay in the U.S, however Zoya doesn't feel that it was right and leaves. He also had a brief relationship with Sylvie Brett, but they broke up not a long time after they started dating.
In reference to episode "Just Drive The Truck".
Joe was the driver of Truck 81, and was suspended from his duties by the Deputy District Chief when 81 got involved in a T-bone collision with Truck 66 from Firehouse 105 in Austin. This is because they were both en-route to the same structure fire. Everyone seemed fine from 81, other than Mouch sustaining an eye laceration. However, two firefighters from Truck 66 sustained broken limbs, and the driver of 66, Jason Molina, was knocked unconscious, and sustained major internal injuries. It was feared that he was not going to recover from these injuries and was in a coma at the hospital. At this point the firefighters from 66 blamed Cruz for Molina's condition, and they thought he would get charged with manslaughter if Molina didn't pull out of it.
At first he was accused of blowing a red light, because he couldn't remember the colour of the traffic light, and because it was caught on a security camera that he blew a red light about 30 seconds before the accident. When Molina woke up however, he confessed that his Lieutenant, Tommy Welch, ordered him to beat Truck 81 so they could take command of operations at the scene - and to do so by any means possible.
He was cleared of the accusation and since Molina woke up, he was not charged. However, he still got a three month suspension from driving the truck and was temporarily replaced by Otis, who was next in line to drive Truck 81. However, Chief Boden managed to reduce his suspension.
In Season 3, it is revealed that he also works as a Zumba instructor (as he does in real life). It was Brett who found out, by attending one of his class after looking on Yelp for the best rating Zumba class, after being encouraged to take up the same things she did before moving to Chicago.
She promised to keep his secret if he accepted to learn how to cook under her guidance - his cooking skills being disastrous. Since Cruz regularly went to talk to her about the class, Casey became suspicious, and went to talk to Brett about romantic relationships inside the firehouse - that they were not encouraged, nor allowed.
To clear the misunderstanding, she revealed to him that Cruz is actually her zumba instructor. Casey talked about to Dawson about it, who told Herrmann, and it spread on until the whole firehouse knew about it. In the next episode, the guys from Truck 81, Squad 3 and Ambulance 61 came to assist one of his classes.
Promotion to Squad 3
In reference to episode "Spartacus"
After Scott Rice is fired because he was ducking out on calls because of his daughter, Chief Boden announces that Cruz received his squad certification. He is given this just before the start of his shift, and he is afterwards given Rice's old spot on Squad 3, just as a call comes in. At first, Otis is upset that Cruz did not tell him first due to the recent tensions between Truck and Squad, because of Rice, but later congratulates him at Molly's.