"Rookie Blue" alum Charlotte Sullivan is headed to "Chicago Fire" season 5. The actress will appear in the winter finale of the season, titled "One Hundred."

Her character is being kept secret at the moment, but executive producer Derek Haas teased to TV Line that Sullivan will play a "woman in distress" who Severide (Taylor Kinney) is "going to make a decision to help out."

The synopsis of the "Chicago Fire" season 5 winter finale teases that Severide will receive an "interesting proposition" from firefighter-turned-medical student Jeff Clarke (Jeff Hephner).

However, it is unknown if this has to do with Sullivan's character and whether or not the actress will be back for more episodes. The fact that she will appear in the back half of the finale gives the impression that it will not be the last time fans will see her.

Meanwhile, Jon Seda, who just left "Chicago P.D." to play his role as Detective Antonio in the spinoff, "Chicago Justice," will guest star in the "Chicago Fire" season 5 winter finale.

It is unclear what he will be up to in the episode, although his presence is not exactly something new in the series as this is where his character's love interest, the paramedic in charge, Sylvie (Kara Killmer), is in.

Seda teased in an interview with TV Line that his relationship with Sylvie affects his sister, Dawson (Monica Raymund). The actor also teased that "there's also some family issues that we'll be looking at."

Elaborating on Antonio and Sylvie's relationship, Seda said, "You have two people who never would have thought they'd find something in common with each other, and yet they find it and the stars align, and you give it a shot."

With regards to what's next for them, he said that he has no idea, "but it's something that is definitely happening, and it's going to be interesting to watch, especially having his sister see her brother in ways that she hasn't seen him in a while."

The "Chicago Fire" season 5 winter finale, "One Hundred," airs Tuesday, Dec. 6 at 10 p.m. ET on NBC.