After Spider-Man cheated death, Spiders ate Peter's old body. The spiders, along with Peter's skin, created a new body. This new body looked female. It told Spidey that she exists because he was supposed to die, so she was sent to kill him. Before much of a battle it escaped from Spider-Man and into the sewer.
During the events of The Other, Spider-Man died fighting his enemy Morlun. However, his body was later discovered as a sack of skin; Spider-Man had shed his skin and had retired to a 'cocoon' under the Brooklyn Bridge, which he later emerged from, completely healed of his prior wounds. Thus, Spider-Man had 'conquered' death.
Meanwhile, in Stark Tower, pirate spiders started to eat Peter's old dead body. When Spider-Man returned, the top of Stark Tower was covered in webbing. Inside the tower, Spider-Man discovered that pirate spiders had consumed Peter's discarded skin. Sated on his flesh, the spiders formed into a body of their own, female in opposition to Spider-man's male.
Declaring itself to be Spider-Man's "Other", the creature explained to Peter that it exists because Peter was supposed to die, but did not. To maintain "balance" the creature had arrived to become his opposite, as well as his equal, and would evolve as he evolved, or die as he would die. It then escaped down a drain. A cocoon was later seen in a church, where the creature presumably hibernated.
Due to the events of the Civil War and the Superhuman Registration Act, Parker decided to register and release his name to the public. Peter was considering leaving the school because of this.
Flash Thompson, not wanting to believe that his idol was "Puny Parker", convinced himself that Peter was merely covering for Spider-Man and goaded him into a dodgeball fight. Peter, no longer needing to protect his identity, made quick work of Flash by kicking the ball into his face and knocking him out.
Flash was taken to the nurse's office, where he met Nurse Arrow ("My parents were inveterate hippies", Arrow said when asked to explain her unusual name). Flash took a liking to her and began flirting. When he asked if she would have problems with the "whole black/white thing", African American Arrow responded, "The world's not black-and-white... just shades of gray."
Later, the school was attacked by the new Mysterio, Francis Klum, who was looking to get revenge on Spider-Man from a previous defeat. Also entering the fray were Daniel Berkhart, the second Mysterio, and even Quentin Beck, the original Mysterio, long thought dead from a self-inflicted gunshot wound after a fight with Daredevil. Miss Arrow found herself in contact with Beck. Beck told her that she seemed to be an innocent bystander, but implied that she was more than that with "'seem' is such a subtle word. As in things are not always what they seem." He went on to mention that he knew about Arrow's 'superiors', and that he could easily see through her charade. He told her that she must work to keep Peter at the school because his superiors wanted it, and so did hers. Once again, Arrow claimed to not know what he was talking about.
Beck later vanished, although Arrow found herself face-to-face with Klum. Klum prepared to hold her hostage, telling that she was to become a pawn in a much larger game. Arrow then revealed her true nature, extending large stingers from her wrists. Telling Klum she was no one's pawn, she plunged one of them into his chest. Before she could finish him off, he teleported.
Arrow later convinced Spider-Man to stay at the school. However, when Peter became a fugitive for going against Iron Man, he had to use an image distorter to disguise himself as "Ben Reilly" to continue his career at the school. Later, Arrow showed more hints that she was not as innocent as she looked; in one scene, she ate a spider behind Flash's back. In another, she acted threatened when Betty Brant flirted with Flash; first she unsheathed her stingers before retracting them and holding out her hand and cryptically stating that Betty "looked so good [she] could just eat her up."
While Flash and Betty were on a date Betty had to use the restroom. While there, thousands of spiders ran out from the toilet, frightening Betty. When Flash and the restaurant manager came to the scene after Betty had left the bathroom, only some cocaine and a straw was found, making the manager think Betty had used the cocaine. Betty and Flash left while Miss Arrow appeared out of the shadows.
On a related note, Spider-Man had earlier met up with Beast at an old church, where they discovered that the Other's cocoon made entirely of webbing was empty; whatever gestated in there had been long gone.
In Friendly Neighborhood Spider-Man #20, Arrow was revealed as the being composed of pirate spiders first seen in The Other. After disguising herself as a human woman, she met Flash Thompson and developed a fixation on him. At the beginning of the issue, she is seen talking to a spider deity in the church where she hatched from her cocoon. It tells her that she is "not long for this sphere" and must reproduce in order to survive. Arrow says that she has decided on a "host": Flash Thompson. In addition to being her host, she declares he will be her mate and then her sustenance.
Meanwhile, Betty Brant visits Flash's apartment, meeting Peter there and figuring out who he is despite his holographic disguise. She then tells him of her recent experiences and her concerns about Arrow. Peter believes she has reason to be concerned, and tells her of an incident at the school. He had been talking to Flash, who also knew Peter's true identity, without his image inducer on. Suddenly Arrow walked into the room, with Peter barely activating the image inducer in time to conceal his true face from her. Peter found it strange that his spider-sense hadn't warned him of Arrow's approach, since in the past it had always warned him when there was a risk of somebody seeing him and learning his secret identity.
Betty and Peter use a search engine to search for "spider" and "arrow". None of the hits they get seem relevant, but Peter notices that the search engine is asking if they meant to search for "spider" and "ero". Peter remembers the pirate spiders that had eaten his shed skin and came together in a humanoid form. He notes that one type of pirate spider is of the genus "ero", which sounds similar to "Arrow".
Arrow, in the meantime, has tracked Flash down at a bowling alley where he has gone with his friend, professional bowler Kelly Kulick, and the bowling team from Midtown High. Arrow insists that the two of them leave immediately for a date, but Flash is reluctant to simply run off and leave Kelly and his students behind. When Kulick becomes annoyed with her behavior, Arrow gives her a violent shove. Flash, shocked, tries to see if his friend is hurt but Arrow grabs him and continues to insist that they leave, telling him she is giving him "a singular honor." Flash is surprised that her grip is too strong for him to break free.
When Flash expresses his displeasure with her actions and tells her to go away, Arrow grabs him by the throat. Before Arrow can drag him away by force, she is attacked from behind by Kulick, and responds by shooting thousands of spiders from her mouth that swarm over Kulick. At that point, Spider-Man arrives on the scene and hits her from behind. During a brief exchange between Spider-Man and Arrow, or "Ero," she tells him that she wouldn't be here if he hadn't "violated the natural order." She says she is his opposite, his nemesis, and his significant other. Then she transforms into a stream of spiders which crawls into one of the gutters and leaves the building.
The spiders that make up Arrow emerge outside, shooting up from beneath the pickup truck Flash and Kelly Kulick have gotten into like a geyser. The truck is propelled high into the air by the spiders. Flash commands Arrow to at least let Kelly go, and she obliges by opening the truck's door and causing Kelly to fall toward the ground.
Kulick is saved from death by Spider-Man, who then swings over to the truck (which has now been lowered to the ground) to deal with Arrow. By the time he gets there, she has escaped into the sewer with Thompson.
Ero takes Flash to the church where she emerged from her cocoon and, after webbing him to the ceiling, removes an eggsac from her stomach. She reveals that she chose him for her mating because he has been in a coma for months, and the fact he hasn't 'had his oats' in all that time means he has built up a tremendous amount of tantric energy. This translates into more offspring for her. Once she shoves the egg sac down his throat, Flash will give birth to thousands of Ero's children, but die in the process.
Spider-Man, remembering the cocoon he had witnessed in the church earlier, tracks Ero down before she can impregnate Flash, plucking the egg sac from Ero's hand and sticking it to a nearby wall. Ero dives from the ceiling to attack him, stingers shooting from her wrists. Spidey leaps to meet her, his own stingers drawn. As they battle, Ero reveals that Spider-Man's stingers only appear when he faces "...someone like me, someone whose being is rooted in primal forces of chaos and darkness." Ero then manages to stab Spider-Man in the shoulder, drugging him and slowing him down.
Having the upper hand, Ero decides to impregnate Spider-Man instead, her logic being that while the offspring will be considerably less than Flash's, it will in the end ensure her enemy's demise.
Just as Ero is about to implant her eggsac into Spider-Man, Betty Brant arrives and blasts the egg sac with silver bullets, fired from a shotgun. Enraged at the loss of her eggs, and vowing revenge against Betty, she tries to escape. Spider-Man follows, and lures her into an aviary, where she is devoured alive by birds. Only a single spider remains, which Spider-Man steps on contemptuously.