Nico is a sorceress like her parents and grandmother before her and can cast nearly any spell imaginable with the mystical Staff of One, using any spell no more than one time.
During their parents' annual "fundraiser," Nico and her friends Alex Wilder, Chase Stein, Karolina Dean, Gertrude Yorkes, and Molly Hayes discover that their parents are members of an archetypal supervillain cabal called The Pride; Nico's parents are the dark magicians of the crime ring. She runs away from home with the other five kids and begins a life in hiding at Chase’s dilapidated mansion hideout called “the Hostel.” In an altercation with her parents, Nico accidentally acquires her mother’s Staff of One and quickly begins to tap her abilities as a sorceress. She expresses feelings for Alex Wilder during their original getaway, but is hesitant to start a romance. She impulsively agrees to date Alex after he saves her from a vampire attack some time later. This pattern of clinging to people during stressful and/or intense situations follows Nico throughout the entire first volume and continues into the second volume. However, Nico proves her loyalty to the runaways when Alex betrays the group and offers her a seat in the new world promised by the Pride's Biblical benefactors, the Gibborim. She refuses Alex's offer and helps usher her friends to safety as the Gibborim slaughter Alex and the Pride; soon afterwards, the gang is apprehended by Captain America and split among different foster homes; Nico is sent to Father Flanagan's Home for Unwanted Goth Kids. However, the group organizes a mass escape from their homes to begin life as full-time vigilantes; Nico is generally accepted as de facto team leader.
The 2007 magic-related crossover event Mystic Arcana fills the gap between Runaways vol. 1 #17 and #18, during Nico's brief time at Father Flanagan's Home for Unwanted Goth Kids. The story begins with Nico dressed in plain clothing, stating that it's perhaps time to let go of the goth trend. She is soon talked into going to a nightclub with her friends, where she meets her parents' old rival Marie Laveau; Laveau seeks revenge on Nico's parents for stealing the Black Mirror, one of the four Cornerstones of Creation. After Laveau explains that she needs Nico's blood to activate the Black Mirror, the two fight, and Nico easily wins. Nico attempts to bring back Alex Wilder using the Mirror, however the spell fails, the Mirror shatters and the last page of the Darkhold is revealed behind the glass. Laveau takes it and flees, while Nico accepts her place as an outcast and with the runaways.
The second volume of Runaways finds Nico taking the leadership position with ease, proving that she has the intelligence and courage to make difficult decisions. After being warned of a future threat named Victor Mancha, Nico and the runaways find and abduct Victor to question him. After Victor witnesses his mother's murder and the rending of his "father," the android villain Ultron, Nico invites Victor to become part of the team, much to the chagrin of some members, most notably Chase. Nico reassures them that she is willing to "rip Victor's damn heart out" if necessary.
Despite Nico's resolve to remain single after Alex's betrayal, Karolina attempts to kiss Nico, revealing herself as a homosexual. Although Nico refuses the advance, she is emotionally distraught when Karolina leaves with Xavin to be part of a peaceweaving marriage, displaying her affection yet again in a stressful situation. Nico also kisses Chase after he negotiates their way out of a potentially fatal situation with an inter-dimensional drug-dealer; this causes a falling out between Nico and Chase's girlfriend Gert, almost dissolving the team from the inside. Gert's untimely death sends Nico into the arms of Victor Mancha, with whom she begins a physical relationship. When Victor asks Nico be his girlfriend, she tells him that between her tendency to seek physical affection as a coping mechanism, her insecurities regarding that tendency, and her survivor's guilt regarding Gert's death, she could not be a steady girlfriend. Nico confronts these insecurities in a dream involving her deceased parents berating Nico for acting like a "slut." She kills her parents in the dream, a symbol of overcoming her insecurities, but awakens bound in a chair at the mercy of Chase, who managed to steal the Staff of One. He reveals his plan to sacrifice himself to the Gibborim in exchange for Gert’s resurrection. Nico interprets this exchange as a cry for help and leads the runaways to intervene Chase's self-sacrifice. The Gibborim do not consume Chase, but instead try to take Nico, revealing that Nico's soul is innocent, thus suitable for the Gibborim's consumption. After defeating the Gibborim, Nico starts a romance with Victor, but soon thereafter leads the runaways in a cross-country escape from Iron Man and S.H.I.E.L.D. superhuman registration forces.
The Runaways travel to New York City and make a deal with the Kingpin; protection in return for stealing an artifact for him. Victor criticizes Nico for siding with the Kingpin, which makes Nico question her leadership abilities. Later, Karolina and Nico get into an argument about Xavin and Karolina's relationship; Karolina angrily flies off, leaving behind a visibly distraught Nico. Later the team breaks into a high-security vault to steal a power source which Chase correctly assumes was made by The Pride. The group is attacked by The Punisher and an older Tristan. Nico casts a spell which allows them to escape, depressed about her poor decisions and the belief that Chase had died in the attack. When the group confronts Kingpin about the device they stole, he ambushes them with ninjas. Nico is severely injured during the fight, but manages to knock out the ninjas with a spell. Trying to escape, the Runaways accidentally time-travel to the year 1907.
Temporarily stranded in 1907, Nico uses the Staff of One to give the group period-appropriate dress. They run into The Swell, who introduces them to the Street Arabs. Victor tells Nico about the message the adult Triston gave him, but she can make no sense of it. The group splits up the next day, under Nico's direction; Nico goes to the New York City Bank to look for another device to get them back home. She has an unfortunate run-in with The Upward Path who knock her unconscious and take her to The Witchbreaker, who turns out to be Nico's great-grandmother. Nico is tortured by The Witchbreaker, who says that the torture will help Nico grow stronger and that Nico has the potential to be great. Because of this, Nico gains new abilities and is able to wield the Staff of One with greater power and efficiency. She returns to the group and takes them to the Leapfrog, stating that the Yorkes are there and can get them back home. Nico casts a spell on the Yorkes to prevent them from alternating the timeline and the group returns to the present. There Nico talks to Chase about his trip back to see Gert, as well as explaining that, in an effort to be a better person, she stepped aside so that Victor and Lillie could be together. The group then decides to go back to Los Angeles.
Nico is a very emotional person and has repeatedly expressed difficulty with knowing precisely what she is feeling. In vol. 2 #19, Nico admitted a tendency to seek physical comfort during or after highly emotional situations or periods of stress. Nonetheless, she is typically very open when trying to express her feelings and has thus been able to connect with many of the runaways on an emotional level and maintain many close friendships throughout the series.
In volume 1, Nico and Alex share a brief relationship. She kisses him for the first time during the runaways' attempted rescue of Molly Hayes in vol. 1 #5, but refuses to talk about it for fear of ruining the moment; it's later revealed in vol. 2 #22 that Alex was Nico's first kiss. She kisses and expresses love for Alex after he saves her from the vampire Topher; it is generally accepted that Nico and Alex are a couple at this point. However, despite her professed love for Alex, Nico chooses to side with the runaways after Alex reveals his plan to bring her to the Gibborim's new world with her parents, firmly establishing her dedication to the team and her strength of character. However, despite the betrayal, Nico demonstrates willingness to forgive when she unsuccessfully tries to resurrect Alex in Mystic Arcana # 4; she states in vol. 2 #2 that although Alex was a traitor, he did not deserve death. She has openely admitted that her feelings for Alex have not changed, she still loves him and says she always will.
Karolina's feelings for Nico were hinted at throughout volume 1, culminating with Karolina sheepishly blushing when Nico says she has sworn off boys in vol. 1 #18. However, Karolina's feelings were not explicitly expressed until vol. 2 #7 when Karolina startles Nico with an attempted kiss. Nico had not shown romantic feelings for Karolina previously and turns Karolina down, saying that she wants time alone. However, when Karolina decides to travel to her home planet Majesdane with Xavin, Nico is distraught and seems to react the strongest to Karolina's departure despite not showing many signs of romantic feelings or even a close friendship during the course of the series. Chase stops Nico from casting a spell to bring Karolina back to Earth, and later catches Nico strewing Karolina's clothes about her floor for remembrance's sake. After Karolina returns, Nico becomes overjoyed and hugs her, saying that she felt "all alone" with Karolina gone. Their friendship is an occasionally strained one, due to Nico's ambiguous feelings towards Karolina and strong dislike of Karolina's fiance, Xavin.
Chase provides the rule-breaking foil for Nico's motherly and leadership role. The pair maintain a strong respect for each other despite often disagreeing over major group decisions, including Gert's burial site and allowing Victor Mancha to join the team. Chase cares for Nico's safety and well-being, as evidenced from when he comforted Nico after Karolina's departure and later when he took the Staff of One from Nico to destroy it (along with himself) and give her a normal life. Nico and Chase shared a kiss after Chase bluffed their way out of a potentially fatal encounter with inter-dimensional drug lord, which Chase rejected. Chase does not make a huge deal of it, even though Nico treats him more as an employee instead of a friend after the incident. After Gert's death, Chase and Nico's friendship strains as he allows anger and grief to isolate him (both physically and emotionally) from the group's support. However, Nico is also the only person that Chase bids farewell before leaving to sacrifice himself for the Gibborim, which Nico interprets as a cry for help, and is presumably a signal of the level of trust and respect Chase holds for Nico. In Joss Whedon's current stint on Runaways, Chase and Nico's friendship appears more stable now that Chase seems to have moved past grieving for Gert.
Gert and Nico had been close friends since birth and Gert had been slated by Nico to take over leadership in the event that Nico was unable to do so. However, they briefly had an altercation when Gert discovered that Nico kissed Chase after he bluffed their way into a drug dealer's good graces. Their falling out caused much friction in the group and threatened to crumble the cadre from the inside. Nico and Gert finally resolved their differences with a short bout of tossing insults and AOL Instant Messenger slang. Despite Gert's cynical attitude, she rarely argued with Nico's decisions and was usually prepared to stand by her friend. Nico experiences survivor's guilt after Gert dies, knowing that Gert's murderer Geoffrey Wilder had initially meant to kill Nico; she begins a physical relationship with Victor Mancha to cope.
After her first sexual encounter with Victor, Nico explains that she seeks physical comfort after periods of stress. While Victor expresses feelings for Nico, she asks for time alone to clear her head and figure her feelings out, much like how she initially dealt with Alex and Karolina after each expressed feelings for Nico. However, Nico admits that Victor is genuinely a good guy and wouldn't mind being in a relationship with him. Victor calls Nico his girlfriend when he attempts to save her from Chase, and although she is in a hostage situation, Nico still lightly scolds Victor for calling her such. By the end of Brian K. Vaughan's run on the title, Victor and Nico are a couple. Joss Whedon's arc introduces a new love interest for Victor, Lillie, a carefree girl from 1907. Trying to become a better person, Nico, seeing that Victor and Lillie are in love, steps aside (albeit a bit grudgingly).
In Brian K. Vaughan's original pitch for the series, Nico was called Rachel Messina. Her parents were still magicians, but posed as wealthy antique dealers; this cover story was ultimately used for Gert's time travelling parents. In vol. 1 #4, Robert Minoru uses the spellbook originally intended to be Nico's instead of the Staff of One. In the same issue, Nico disbelieves that her parents are sorcerers, citing that they take her to church every Sunday. In the original story, Nico was to be a devout Christian, inciting an internal clash between her devotion to Christianity and the necessity to use dark arts for survival. In vol. 2 #9, Vaughan reveals that Nico is a former altar server, a throwback to his original plans for the character. Later, Nico tells Victor that her parents had raised her to be religious, but she quit her faith after finding out their identities as dark magicians. After Molly is kidnapped by the New Pride however, she confesses that she began to pick up on her Christian beliefs since their parents deaths as the Giborim were real, so maybe the rest of the Bible was real as well; she also confesses to Victor that she hopes that there is a "better place" if they die.
It is believed that Nico cannot say the same spell twice. However, on Murder Island in Avengers Arena, she was able to cast the same spell twice by using "Chill Out." (A spell she had used in the past.) However, she did mention that her spells on the island is a bit more wonky then usual.