Nicolette Riordon was formerly the mayor of Defiance and one of it cofounders.
Nicky was born an Indogene, but camouflaged herself as a human while she searched for the Kaziri, a hyper-advanced sentient computer created by Indogenes 3000 years ago to terraform the earth. Nicky dedicated her life to finding the Kaziri because she wanted to protect the planet for both humans and Votans.
Early Days of Defiance
Nicky was working as a Clerk for the Earth Republic when she read about the miraculous survival of the Gateway Arch after Arkfall. She believed the Arch’s survival was a sign that the Kaziri remained buried under St. Louis. She approached Colonel Marsh with her theory, but he didn't believe her, so, in 2033, Nicky and a team disguised themselves as miners and led an expedition to Missouri to find the Kaziri. 
This expedition revealed that a massive vein of gulanite existed below St. Louis. A race for resources inspired a mass immigration to the area where Rafe McCawley opened his mines and earned his fortune.
Nicky settled into the newly created mining town where she and Datak Tarr procured lithium to help treat Rafe's wife, Pilar's, bipoler disorder. Nicky claimed to have been a quartermaster in the war which gave her the means of obtaining challenging items. Eventually, the lithium, which was more than a decade old, stopped working, and Pilar attempted to kill her children, but Nicky helped persuade Rafe not to kill Pilar in retaliation. Nicky was having an affair with Rafe at the time, and was therefore able to persuade him. She knew he could never be happy if he murdered the mother of his children.
Nicky and Rafe officially cofounded Defiance in 2037, when it was still a small mining town. Publicly, she told the townspeople she wanted to create an inclusive community where everyone could start over, but she had an ulterior motive. She was dedicated to finding the kolovan and the Kaziri.
After founding the town of Defiance, Nicky became mayor around 2038 to govern the expanding population.
When Meh Yewll first arrived in Defiance looking to start over and change her life, Nicky told her everything she wanted to hear. She inspired her, and Yewll became friends with Nicky and Mr. Birch. The three worked together until Yewll grew frustrated with their reckless behavior.
Around the time Defiance was officially founded, Amanda Rosewater moved to the town where she worked as a janitor, scrubbing floors at the Darby building. Nicky saw her one night and said, "honey, you're scrubbing that floor like it's the most important job of your life." Then she made Amanda her new assistant. Even though Amanda was a nobody in Defiance, Nicky treated her like an equal.
In 2038, Mayor Nicky attempted to forcibly inoculate Irathient children, resulting in a riot in which many Irathients died and even more fled. This greatly reduced the Irathient population in Defiance.
In 2039, Nicky and Doc Yewll killed Hunter Bell after he walked in on Yewll examining Nicky and discovered Nicky's secret Indogene Identity. Hunter tried to blackmail them for 100G to keep the secret, but Nicky killed him instead. Yewll shouted, "you psycho piece of shtako! You chuping killed him!" but Nicky simply replied, "I was afraid I hadn't hit him hard enough." Jered Kikema then walked in and caught them, but he never told anyone what he saw.
Back on their homeworld, Castithan children would often be segregated according to their liro, with students of the shanje liro enjoying a better education. Nicky put a stop to this. She saw this practice as unfair and outmoded, and integrated Castithan children along with the rest of the Votan races. This led to conflict among Castithan children and parents alike, an unfortunate consequence of Mayor Nicky's attempts to unify the town and its people.
During her time as mayor, she also set up a fund to restore the arch. Renovations were ongoing throughout the first two seasons, and only one of the original two elevators had been restored to working order before Stahma and Datak Tarr blew it up in season 3.
After serving 8 years as the mayor of Defiance, Nicky appoints her assistant Amanda Rosewater as her successor until elections can be held. Known as Mayor "Nicky" she is well loved. Amanda doubts that she can fill Nicky's footsteps, but Nicky insists that she has an eye for talent and that Amanda is a natural leader. Despite her popularity, Nicky cites health problems as one of her reasons for resigning. She uses a portable oxygen canister that suggests she suffers from a respiratory condition; however, she occasionally puts it aside to smoke.
She states that she plans to spend time with a niece who lives not far from Defiance in the town of Cedars, but shows little enthusiasm for it. She complains that the "Earth Republic has drained all the color out of that place" and advises Amanda not to enter into financial or trade agreements with them because they will try to do the same to Defiance.
In the pilot episode, Nicky is on stage when Amanda gives her speech dedicating the statue to the Defiant Few on the anniversary of the armistice that ended the Pale Wars. Amanda thanks Nicky for commissioning the statue. After finishing her speech, Amanda encourages Nicky to stay, but Ben Daris later informs Amanda that Nicolette left Defiance for Cedars on the ground coach as scheduled.
When news of the coming Volge attack frightens the townspeople, Amanda goes to talk to them. Many in the crowd at first ask "where is Mayor Nicky?' and "where is the real mayor?", but Amanda is able to win them over.
Search for the Kaziri
After the Volge are defeated, Nicky meets Mr. Birch at Cafe Renaldo. He tells her that that their plan has failed. The Volge were defeated, the acting mayor is more capable than anyone gave her credit for, and their co-conspirator, Ben Daris, is in a coma and could betray them if he wakes up. The details of her plan are unclear, but Nicky is involved in a conspiracy that will force all the residents of Defiance from the city one way or another. She reminds Mr. Birch that they do have a plan B, but Birch says that it will be brutal. Nicky insisnts that it must be done and that the "survivors will thank us."
In Down in the Ground Where the Dead Men Go, Nicky clears her things out of her former office, telling Amanda it's time for her to leave her own mark. As she clears out her things, she discusses Castithan culture with Amanda who wants to stop Castithan shaming rituals. Nicky warns her that if she tries to stop them, it will be just like the situation with the Irathients all over again, but Amanda insists Defiance has changed a lot in the last five years and that Votans and humans are finally coming together. Nicky smiles, straightens Amanda's braid and tells her that she will miss her.
As she puts her things in the car, Mr. Birch tells Nicky that she let herself get too close to the people of Defiance, but Nicky retorts that she grew up with them. She insists her actions are for the benefit of all and they must be done, but she takes no pleasure in them.
In Brothers in Arms, Nicky comes up on Quentin in Cafe Renaldo to ask him about the symbols that appear both in the book he's reading and in the mines. During this conversation, she confirms that Quentin has one of the kolovan, the artifact that she has been searching for, and that he got it from his brother. When Quentin discusses this incident with his father, Rafe McCawley, he tells Quentin to get rid of the artifact because Nicolette Riorden is a dangerous woman. Quentin does not listen to his father. He keeps the gold kolovan, and when Mr. Birch breaks into his house to try to take it from him, Quentin murders him after seeing a vision of his dead brother, Luke, encouraging him to do so.
After Birch's disappearance, Nicky goes to the McCawley house to look for him. She tells Rafe she thinks he killed Birch. Nicky then breaks into a coughing fit and Rafe tells her she can stay in Pilar's sewing room. She already knows where it is. After finding out that Quinten killed Birch, Rafe lies to Nicky and confesses to killing Birch for trespassing. Nicky realizes it's a lie because she knows he never would have admitted to killing Birch if he had actually done it.
In If I Ever Leave This World Alive, Nicky once again meets Quentin in Cafe Renaldo. Quentin asks her if she should be smoking given her breathing difficulties, but she claims, "Oh, cancer is so slow. Something far more interesting is bound to kill me first." She reveals to Quentin that his mother is not dead, and that his father lied to him. She promises Quentin that she will tell him how to find his mother if he brings her the gold kolovan. She also reveals she knows that he killed Mr. Birch to protect it. The Kaziri, still appearing to Quentin as Luke, tells him not to give her the artifact - if he gives it to her, he'll never see "Luke" again, but Quentin gives it to her in exchange for the information about his mother. She tells him that Pilar is in Mendocino, and takes his keycard to gain access to the mines.
Death & Yewll
Later, Doc Yewll meets Nicky in her car, taking Nicky's cigarette from her. Yewll criticizes Nicky for the Volge attack and for triggering a radiation leak in the mines. She insists that Nicky and Birch were always reckless and stupid, which is why she stopped working with them, but with Birch's death, Yewll and Nicky are the only two who remain of the old gang. Nicky attempts to use the fact that she's obtained the gold kolovan to persuade Yewll to work with her again, but Yewll tells her to destroy it immediately. Nicky continues to beg Yewll to help her reshape the planet, but Yewll is worried Nicky will actually destroy the planet. Nicky simply replies, "you can't create without destroying. I thought you would have figured that out by now, Doctor."
After Hunter Bell's body is found, Doc Yewll summons Nicky to her office. Nicky excitedly thinks that Yewll has decided to work with her, but Yewll wants to talk about a Liberata bartender, Jered Kikema, who was recently killed. Jered was the only person who knew Yewll and Nicky had killed Hunter Bell, so Yewll comes to the logical conclusion that Nicky killed the bartender to keep him from talking. Realizing both she and Nicky have changed since the days when they were friends, Yewll kills Nicky. She knows Nicky has been corrupted by her mission to find the Kaziri, and therefore cannot trust her with it. Yewll says, "If I can't trust you with a simple walking stick, how am I supposed to trust you with a genocidal weapon?"
Yewll writes a suicide note for Nicky in which "Nicky" confesses to Hunter Bell's murder. The suicide note reads, "Today is a special day, an important day for Defiance. That's why I didn't want to spoil it with my presence. I killed Hunter Bell for what I thought was the betterment of this town. When his body was recently discovered, I killed Jered Kikema, whose only crime was witnessing my own. I realize it's only a matter of time before the lawkeeper discovers the truth. With my health, I'll never survive Vegas prison. So I've decided to end my life. I told myself I was acting for the greater good. But that kind of thinking leads you down a dark road. I can only hope I've left the town I love in better hands than my own. I'm sorry."
Nicky was cold and calculating in her mission to find the Kaziri and reshape the planet. She would stop at nothing, including murder, to accomplish her goals, but she also grew to genuinely like the people of Defiance and took no pleasure in hurting them. And they liked her in return. To quote Amanda Rosewater, "She had a way of making you feel like you were the most important person in the room." She had a lot of friends and supporters, but very few of them knew her true nature.
- Yewll says it happened 7 years before season one which is set in 2046