A Nemesis is two people who are enemies beyond the normal spectrum. A Nemesis can be the most hated foe, or the one who has the most closely tied-in backstory. In the Heroes universe, their are four known Nemesis relationships:
Niki and LindermanEdit
Nicole Sanders, despite her continuous attempts at simply making ends meet and taking care of her son, was constantly antagonized by the gang boss Daniel Linderman during season one. Niki had taken several loans from Linderman, which she failed to pay back. When Linderman sent two thugs to collect, Niki's psychotic alter-ego, Jessica, killed them.
When Niki fled to Las Vegas, Linderman, rather than continuing to hunt her, manipulated her, and made use of her as an Assassin(Jessica) and otherwise representative/agent (Niki). When Niki was committed to an Asylum due to Jessica's presence, Linderman's men broke her out so as to continue to use Jessica as an assassin. However, Niki saw this as a curse, as Jessica had been set loose on the world once again.
As Jessica continued to work for Linderman, she and Niki engaged in a vicious battle of the wills for control over her body and life. When Niki finally conquered Jessica, she teamed up with her husband, D.L. Hawkins, and together, the duo fought through Linderman's building, retrieved their kidnapped son, Micah, and killed Linderman.
Hiro and AdamEdit
During season one, Hiro Nakamura made multiple references to favorite children's story, the tail of legendary samurai, Takezo Kensei. At the end of season one, Hiro killed Sylar with his weapon of choice, Takezo Kensei's sword. However, with his last bit of strength, Sylar attacked Hiro with telekinesis, throwing him towards a nearby wall, causing Hiro to reflexively teleport to safety.
Lacking full control over his powers, Hiro accidentally teleported back 409 years, to fuedal Japan, where he met his hero, Takezo Kensei. However, to his disdain, Hiro found that Kensei was not what he had expected: rather than a proud, bold, noble samurai warrior, Kensei was a drunken, two-timing, greedy, non-heroic englishmen. He also discovered that Kensei had healing powers. Upon finding this, Hiro was able to temporarily convince Kensei to be a true hero.
However, when Hiro betrayed Kensei's trust by kissing the woman he loved, Kensei was quick to return the betrayal, capturing Hiro and preparing to join forces with a tyrant's army, using a shipment of guns to take over Japan, and "change history". Hiro escaped and confronted Kensei in a super-powered swordfight. In the end, Hiro was forced to leave Kensei to apparently die in a massive gunpowder explosion, but not before Kensei swears that he will destroy all that is precious to Hiro. Hiro then teleports back to his own time, realizing that he had fulfilled the legends of Kensei, rather than the real person.
In the future, Hiro's father has been killed, and Hiro is shocked to learn that the killer is Takezo Kensei, not looking a day older after 400 years (due to his ability), and now a villain under the name of Adam Munroe. Seeking revenge, Hiro goes to kill Adam, but is thwarted by the misguided Peter Petrelli, who is truly a hero who has been tricked into an alliance with Adam. Being far more powerful, Peter defeats Hiro, and he and Adam continue on their objective, which Peter thinks is to destroy a deadly virus, though Adam truly intends to unleash the virus, wiping out the entire human race.
As Peter is distracted by Nathan Petrelli and Matt Parkman, Hiro confronts Adam, who realizes cannot be killed. Therefore, he instead decides to teleport Adam into an empty casket underground, trapping him forever.