A Chicago Fire character accidentally killed a man (you won't guess who)

The promo and synopsis for the Chicago Fire episode "On the Hook" really had us worried. It led us to believe that Sylvie Brett (Kara Killmer), the excellent paramedic who is on the cusp of getting married, was going to make a mistake that would take somebody's life.

Brett was the character referenced in the episode's title, and most of the runtime was dedicated to the character's desperate attempt to clear her good name. She was able to do so, thankfully, and the person who suffered at the hands of the defunct defibrillator wound up being ok.

Derrick Gibson has tried to hide his past from 51

Things went less smoothly for the newest member of the 51, however. Derrick Gibson (Rome Flynn) has proven to be a good firefighter, but he's also been a bit of a cypher; unwilling to talk about his life or get close to his co-workers.

Well, we found out why in "On the Hook." Gibson opened up and voiced unhappiness with the fact that the 51 googled his name prior to hiring him, and constantly asked him questions about what he used to do.

Related: Sylvie Brett finally chose her maid of honor

Sam Carver (Jake Lockett) did most of the prying in this regard, but he came to realize that there was something else going on with Gibson, and he was right. As the episode wears on, the the innocuous circumstances that led to a basketball player losing his pulse came to light, Gibson dropped a major bombshell about his past: He accidentally killed a man.

Gibson used to be a professional boxer, and he told Carver that things took a severe turn during one of his bouts, leading to the death of his opponent. The guilt and trauma that resulted led to Gibson hanging up his gloves.

He told Carver he accidentally killed a boxing opponent

It had been announced that Gibson was an ex-boxer when the character was first cast, but nothing could have prepared us for the revelation that he carries around the guilt of taking another person's life.

Carver doesn't say much after learning this information, but it's safe to assume that these two men are going to bond over the course of season 12, and lay the groundwork for what will be the next generation of 51 firefighters. Both guys have had their fair share of trauma, so we can see them getting along well the more they work together.

It will also be interesting to see how long it takes Gibson's past to reach the other members of the 51, and whether it will affect the way that some of them perceive him. Either way, the character just got a whole lot more interesting!