Nechayev was a significant figure in Starfleet's dealings with the Cardassian Union and a fierce advocate of Federation security. She was Captain Jean-Luc Picard's direct superior but had a poor working relationship with him.
In 2369, while serving as a Vice Admiral, she ordered Picard to relinquish command of the USS Enterprise-D to Captain Edward Jellico, the latter having experience with Cardassians in the past and having worked to establish the original armistice of the Federation-Cardassian War. She assigned Picard to a special operation to infiltrate a Cardassian installation on Celtris III. After Jellico's negotiations with Gul Lemec worsened, she authorized his actions against the Cardassian warships in the McAllister C-5 Nebula, at the risk of provoking open war and abandoning Picard. (TNG: "Chain of Command, Part I", "Chain of Command, Part II")
Nechayev was promoted to the rank of Fleet Admiral in the year 2370, during which time she paid another visit to the Enterprise-D. Nechayev believed that Picard was too soft-handed in his dealings with Federation adversaries. She cited in particular his refusal to use Hugh as a carrier to introduce an invasive program into the Borg Collective. When she later commanded the Starfleet task forces assembled in response to the Borg attack of 2369, she ordered Picard to deploy the program should another opportunity avail itself. Picard tried making amends in 2370, offering her Bularian canapés to help her feel welcome aboard the Enterprise. However, his objections to her order removing the Native American colonists from the Dorvan V colony in the Demilitarized Zone left tension between the two. (TNG: "Descent", "Journey's End")
During this year, she also played a role in the Federation-Cardassian Treaty, and subsequently oversaw Starfleet's response to the Maquis insurrection following Gul Dukat's kidnapping. Though she understood and sympathized the Maquis position, she was committed to preserving the Federation's peace with the Cardassians despite Central Command arming their colonists covertly. As part of her efforts, she requested Ro Laren's participation in an undercover mission to infiltrate the Maquis. (DS9: "The Maquis, Part II"; TNG: "Preemptive Strike")
Nechayev was used in a simulation performed by the Vorta Borath to investigate possible reactions of Benjamin Sisko, Jadzia Dax, Miles O'Brien, Julian Bashir and Subcommander T'Rul to an increased Dominion presence in the Alpha Quadrant in 2371. In this simulation, Nechayev informed Sisko that Deep Space 9 was being handed over to the Dominion, an agreement the Bajorans strongly objected to. (DS9: "The Search, Part II")
Later that year, when the crew of Deep Space 9 had recovered a Jem'Hadar youth, and Odo opted to return him to the Dominion rather than to hand them over to the incoming USS Constellation, Sisko justified the change of plans by stating that while Admiral Nechayev won't like it, "I would've had to kill the boy to keep him here." (DS9: "The Abandoned")
"Admiral, with all due respect, it's not necessary to give Captain Jellico command of the Enterprise just to conduct a negotiation."
"I disagree. The Enterprise will be in a dangerous situation and I want someone on the Bridge who has a great deal of experience with the Cardassians. No offence, Commander, but that's not you."
"Your priority is to safeguard the lives of Federation citizens, not to wrestle with your conscience."
"Granted, but to go to them now after twenty years later and ask them to leave what is now their home."
"I made that same argument with the Federation Council. But it took three years to negotiate this treaty. Some concessions had to be made, and this is one of them."
"Evek manages to make the Cardassians sound like helpless sheep being preyed on by Federation wolves."
"The Maquis are a bunch of irresponsible hotheads."
Nechayev was played by Natalia Nogulich. She wore the rank insignia of a vice admiral in all of her appearances.
In her first on-screen appearance, the script for "Chain of Command, Part I" describes Nechayev as, "a tall graceful woman in her late fifties, with a dark complexion. She charges into the room, her every movement emphasizing urgency." This would put her birth year at some point during the 2310s. 
In an interview with SubspaceCommunique.com, Nogulich revealed she was given the part of Nechayev after interviewing for two different roles on Star Trek: The Next Generation. Though she was unsuccessful in the auditions, the producers were impressed by her work and later offered her the role of Nechayev. As Nogulich recalled, "They liked my work, they seemed interested in me, but they said I wasn't quite right for that particular role, or whatever it was. [...] And then I went in the second time for something else, and it also wasn't a complete fit. But again, they were clearly interested in having me be, you know...when they ask you back to audition more than once, it's a sign that they're appreciative of your work. [...] So anyway, finally, the third time, my agent said, you know, you're going for this role which will probably recur and it's a very strong part...and they had actually gotten feedback about me from the other roles, I was too strong for whatever the other roles were. So this one is a really strong character and you can go for it and they seem to think it's a good match for you. So, I really dug my teeth in for the audition and worked very hard on it, and went in very prepared. It just felt great when I did it. It felt like I nailed it and indeed, what was a day later, or later that day, I'm not sure, I got the call from my agent that I was going to do it. And then I ended up doing another, and another, and another." 
Nogulich came to believe that, had The Next Generation gone into another season, her character would have been developed even more with Picard so that their relationship wasn't as adversarial. Looking back on playing Nechayev, she recalled, "It was a perfect fit. I mean, I have a slavik background, she had a slavik name. I do tend to play strong characters. I'm a character actor and that was a good fit."  
When playing Nechayev, Nogulich was influenced by the experiences of her family, many of whom had served in the military. She drew particular influence from her father who had served in World War II, taking part in the Normandy invasion; and her grandfather who was a Congressional Medal of Honor recipient. "So having this background of, you know, military," Nogulich explained, "I did look to that, what I had learned and observed from stories from them, and then...I realized what she was saying and realized this woman, Nechayev, was passionate about principle. She really believed in what she was saying was the absolute best thing for the Federation. And I got from her, like most leaders, she had to make hard decisions that were not popular. She couldn't afford to be the humanitarian that Picard was. She couldn't afford to say, 'well yeah, maybe the Borg are people too and we should be nice to them', you know. They were our enemies and she needed to the be the one who said, 'next time, you have an opportunity to destroy them, you will do it'. [...] So, I guess when you ask me who historically...I really look to my dad and my grandfather." 
In a 2013 interview with Ellis Martin, Nogulich recalled a US Marine approaching her during the Official Star Trek Convention in Las Vegas and telling her of generals who watched her performance as Nechayev in order to get an idea of how to speak to their subordinates. Though she was flattered by the comment, Nogulich commented, "It all goes back to the writing, I have to bow my head to that. Because they wrote a great character. And they wrote worthy conflict between me and Captain Picard. And also between me and Captain Sisko - Avery Brooks. [...] And perhaps it was fueled by the fact I was a woman, I don't know." 
Writer Ronald D. Moore decided to utilize Nechayev as a recurring character due to both continuity of the Maquis arc, and as a reflection on Nogulich's work. In "Journey's End", Moore initially wrote Nechayev as resisting Picard's overtures of détente but later agreed with Michael Piller that Picard could actually get through to her. (Star Trek: The Next Generation Companion, p.291)
Director David Livingston later mirrored these remarks, commenting, "You want to have the opportunity for crossover and continuity. It's fun to do that with a character when you find somebody that's good in the part, like Evek and Admiral Nechayev. The audience enjoys it." (Star Trek: Deep Space Nine Companion, p.134)
The Pocket TNG novel The Genesis Wave, Book 3 gives her year of birth during the mid-2300s. In the first book of the series, Nechayev is appointed lead Starfleet Officer in the Genesis Wave Crisis. Personally seeing the evacuation of many planets, and with the crew of the USS Enterprise she is able to prevent the wave from striking Earth. However, her face was temporarily disfigured when she is on a planet hit by the wave. Although protected she is given newly mutagenic soil which heals her burns and saves her life, giving an almost two-face effect on her appearance. Nechayev later has surgery preformed to restore her face to normal.
In the Star Trek: The Next Generation - The Space Between comic An Inconvenient Truth, Nechayev enters Starfleet Academy determined to prove herself. Years later, Academy groundskeeper Boothby describes her as having, "a chip on her shoulder the size of Jupiter, so ready to prove a beautiful woman could be a starship captain". Later in the story, Nechayev becomes involved in a conspiracy within Starfleet, which had been using discoveries from the logs of Starfleet vessels to create weapons and forward a political agenda in the Alpha Quadrant. However, she is not totally compliant with the conspiracy - while she promised to "handle Picard", she saves his when an assassin within the conspiracy tries to kill him, and in the process beams the Captain into the conspirator's base of operations.
Nechayev also appears in the Pocket DS9 novel Day of the Vipers, which states that, by 2328, Nechayev was a lieutenant in Starfleet Intelligence. That year, based on intelligence from Bajoran dissident Keeve Falor, she traveled to Bajor disguised as a Bajoran under the name Nechen Alla, to investigate claims that the Cardassian Union was planning to occupy the planet. With her partner Gwen Jones, she was able to discover evidence of these plans, but upon returning with the intelligence to the USS Gettysburg, learned Starfleet intended to treat the matter as an internal political affair. Nechayev later informed Bajoran refugee Darrah Mace that she would do all she could to help the Bajorans resist the Cardassians.
According to the Pocket TNG novel Rogue Saucer, by 2342, Nechayev had achieved the rank of Commander and was assigned to lead a small team on the Ganymede colony. This is contradicted in the novel A Time to Be Born, which states Nechayev had already achieved the rank of Rear Admiral by 2341. In this story, Nechayev meets with Data, who was hoping to join Starfleet and allowes him to skip the Academy entrance exams and to move to an accelerated track with credit for individual study. While in A Time to Heal, Admiral Nechayev is one of five Starfleet admirals who, along with Captain Jean-Luc Picard and Ambassador Lagan Serra, decide the fate of Federation President Min Zife after the disastrous Tezwan affair in 2379.
Furthermore, in Rogue Saucer, Admiral Nechayev orders Captain Picard to test a new Galaxy-class saucer section while the Enterprise's saucer section undergoes repairs. She accompanies the Enterprise crew during these tests and later confronts the Maquis when they attempted to hijack the saucer section, subsequently crash landing it in the ocean of a nearby planet where it is destroyed.
The Star Trek: New Frontier novel House of Cards states that, by the early 2360s, Nechayev had attained the rank of Vice Admiral and oversaw operations at Starfleet Intelligence. In 2370, she assigns Mackenzie Calhoun to do undercover work on the frontier, after Calhoun resigns from Starfleet. She continues to give him new assignments, but after a few years, becomes worried that he is becoming corrupted by the nature of his work. In 2373, Nechayev controls key aspects of Starfleet's response to the collapse of the Thallonian Empire. After being convinced by Captain Picard, Commander Riker, and Ambassador Spock, she decides that the best course of action is to send a Starfleet vessel, the Excalibur, into Sector 221-G to observe the situation and offer humanitarian assistance. She asks Picard to retrieve Mackenzie Calhoun from his operative work to command the Excalibur.
This is followed up in the Double Helix novel Double or Nothing, in which Nechayev, monitoring the Double Helix incident, installs Commander Riker as the field captain of the Excalibur, pulling Captain Calhoun from the ship and sending him on another mission related to the Double Helix through her connections at Starfleet Intelligence.
She later appears in the Star Trek: Terok Nor book Dawn of the Eagles, when she is contacted by Elias Vaughn regarding a Cardassian informant, Gaten Russol, who wants to give information about the occupation of Bajor. She reassigns Vaughn to Starbase 375 so that he could make contact with Russol, but gather only information relating to the Federation and not discuss Bajor. This takes place in the year 2365.
The Strange New Worlds II short story Seventh Heaven reveals that Nechayev's orders to destroy the Borg at the next available opportunity were disseminated to all Starfleet commanders.
In the Invasion! novel Time's Enemy, Nechayev participates in a subspace conference with Admirals Judith Hayman, Hajime Shoji and Kirshbaum, Captain Benjamin Sisko and Professor T'Kreng of the Vulcan Science Academy about a possible Fury invasion. The other purpose of the discussion is the discovery of the USS Defiant frozen in Earth's Oort cloud. Nechayev suggestes that the present Defiant be kept in drydock at Deep Space 9 until the future version disappears. This story takes place in 2371.
Nechayev briefly appears in the Deep Space Nine novel Hollow Men, in which she engages in a heated debate on the Dominion War with Tomas Roeder in a Federation News Service broadcast, and according to the Pocket TNG novel Before Dishonor, she witnesses the destruction of Pluto by a Borg supercube from Starfleet Command on Earth.
In Greater than the Sum, Nechayev informs the crew of the Enterprise that a Borg invasion of the Alpha Quadrant has begun in early-2381. This story is continued in the Star Trek: Destiny novels Gods of Night, Mere Mortals and Lost Souls, in which Nechayev plays a prominent role.
The SCE eBook Progress states that when Nechayev reached the rank of Captain, she was given command of the USS Boudicca. Her first officer on that ship was Commander David Gold. In the follow-up book The Future Begins, a displaced Captain Montgomery Scott encounters Admiral Nechayev on the pleasure planet Risa in 2375.