Cissnei first introduced herself to Zack when he made a remark about SOLIDER outsourcing to the Turks to which Cissnei retorted that SOLDIER's being stingy. When Zack offered to help out the Turks, Tseng immediately declined his offer but Cissnei interrupted, commenting on Zack's generous offer with a hint of sarcasm in her comment (though she was probably half-joking, along with the stingy comment earlier).
When Cissnei was having trouble with a more advanced Genesis Copy later on, Zack came to her rescue. As the copy loses the battle and lay on the ground, it sprouted a black wing trying to escape. Zack struck it down with a grim look on his face, possibly due to what happened earlier in Banora Village with Angeal and Genesis. Cissnei proceeded to tell Zack her childhood dreams of having wings. When Zack commented that people with wings are monsters, Cissnei protested that wings symbolize freedom rather than monsters, which seemed to have cheered Zack up a little.
After the events in Sector 8, Cissnei reported back to Veld, as seen in the special Reno episode in Before Crisis, telling him that Zack and Sephiroth had joined up for a new operation. Veld commented that he had not heard about this new operation from Director Lazard and proceeded to tell Cissnei to keep track of Zack's movements.13 The fact that Cissnei has taken on an assignment to keep Zack under surveillance was mentioned in the timeline of the Crisis Core Ultimania14, but it did not explicitly state who gave Cissnei that mission.
Although Zack is the target of Cissnei's surveillance assignment, and an escaped subject from Hojo's lab that must be captured by order of Shinra later on in the game, to Cissnei, Zack is a colleague (albeit from a different department within Shinra), a genuine friend outside of the Turks members, and perhaps even an inspiration to her.
Throughout the game, Cissnei tells Zack bits and pieces of her past, some of which are probably very personal to her, like her childhood dreams, and the fact that she never thought anyone would celebrate anything she did. Eventually, she even apologized and confessed to Zack that she was on assignment to keep Zack under watch while they were “vacationing” in Costa del Sol both in an email and in person. Her apology in person could be viewed in a DMW sequence, which took place after the mission in Junon. After she apologized to Zack, he told her not to worry about it, as he understood that she is a Turks member.
Cissnei also has a tendency to tease Zack when the two engage in conversations, even pointing out Zack's naivety at times. For example, when the two were at Costa del Sol, Zack was frustrated due to being forced to take a vacation and was doing squats at the beach when Cissnei showed up and asked him if he wanted any Suntan lotion. When Zack later told Cissnei to go do something else to give him some privacy, she immediately asked him if he was going to call Aerith, much to Zack's surprise. Later on, when Cissnei found Zack in Gongaga, she told Zack that he was predictable in that he did not stop to think that his hometown would be the first place Shinra would look in order to capture him.
As a friend, Cissnei also looks out for Zack's well-being, as evident in an email to Zack that she wrote:
Subject; You're not busy, are you?
I guess SOLDIER is in a state of utter chaos without Director Lazard.
It's certainly more than the president can handle, so now he gives us Turks all the work.
Even just the other day, I had to cut my vacation short.
Well, the last one... I guess you know all along that I was on assignment. I'm sorry.
How are you doing these days? Still smoldering, I bet.
Why don't you try the beach again?
You need to take a break once in a while if you want to put in quality work.
In the end, when Cissnei's assignment became to track down and recapture Zack and Cloud as escaped test subjects, she was hesitant in fighting Zack when she first located him alone on the beach near Nibelheim. She eventually let Zack and Cloud escape after seeing the condition Cloud was in when she relocated them again, telling Zack to get away safely, even going as far as giving Zack a parting gift (a motorcycle to help him make his escape) as she walked away. When she met Zack in his hometown of Gongaga while Zack is still supposedly on the run from Shinra, Cissnei gave him time to make his escape yet again.
It is mentioned in the Crisis Core Ultimania that after meeting Zack in Sector 8, Cissnei was drawn to Zack's humanity, and by interacting with him and her Turks comrades, she gradually begins to understand the importance of the bonds between people.15