The legend of Takezo Kensei
The name "Kensei" means "Sword Saint" and "Takezo" is the birthname of the historical swordmaster Miyamoto Musashi — Kensei is based upon various legends of Japanese swordsmen, including Musashi. According to the legend, Kensei was a Japanese hero, well known for saving Japan from "White Beard" (白髭, Shirohige?) in the 17th century. However, he proves to be an Englishman who came to Japan seeking to get rich.
Hiro mentions the legend of Takezo Kensei many times in season one, though little is actually related until he trains in kendo with his father in the episode "Landslide". Previously, in "Godsend" and "The Fix", Hiro mentioned that the sword — which he was working to steal from Sylar in New York, Hiro relates the story of "Kensei and the Dragon" and realizes that, if he wants to win, he must be strong enough to sacrifice himself.
In the 17th century, civil war threatened Japan; behind it was the warlord "White Beard." Kensei found a sword frozen in the snow. He was not, however, a good enough fighter to wield it, so he went to see the dragon of Kiso Mountain, who could teach him the sword's secrets. The dragon agreed to do this in exchange for Kensei's love. Knowing that Japan would fall under the rule of "White Beard" unless he took action, he had to agree to the dragon's demands even though he loved a princess.
After saving Japan in a great battle, the dragon came to Kensei to claim his end of the bargain. The dragon demanded the life of the swordsmith's daughter, the princess Kensei loved. Instead, Kensei cut out his own heart and handed it to the dragon, saying, "My love is in here. Take it," and he died.
According to the mockumentary "Sword Saint", presented on the official Heroes 360 website, the dragon was touched by how much the warrior was willing to offer for his people and love, and restored Kensei's heart to him. Kensei then ran away with the princess.
 From zero to hero
After Hiro time travels to 1671 in %28Heroes%29” title=”Lizards (Heroes)”>Lizards", Hiro, unable to convince Kensei to become a hero on his own, disguises himself as Kensei and creates one of the first stories himself by disarming twelve bandits. In doing so, he enamors Yaeko to Kensei, which in turn convinces Kensei that becoming the legend might be beneficial. When the bandits return for a sneak attack on Kensei, firing several arrows into his chest, it is revealed that Kensei possesses the power of cellular regeneration. Though he seems to die at first, his wounds heal and he revives as Hiro removes the arrows. Kensei is visibly surprised by the turn of events, suggesting that he had been unaware of it prior to his encounter with Hiro.
Initially, Kensei is horrified by the discovery of his ability, believing Hiro to have cursed him. Once Hiro convinces him that it is a natural gift, he is delighted. Since he is functionally immortal, he can take on any challenge without having to worry about dying; in particular, the prospect of wagered duels seems profitable. Still dismayed that Kensei is unwilling to accept his destiny, Hiro forces Kensei to live out another story: retrieving the fire scroll from the 90 angry ronin (also called the 90 Hungry Ronin in Japanese folklore). All the while, Hiro reassures Yaeko that Kensei will return, which he does, now willing to fulfill his destiny.
The three then set to work on freeing Ōtsu from White Beard, only to find that the conqueror's entire army has settled there. Having gathered a map of the camp, they are able to sneak in and rescue Yaeko's father, only to discover that, during his captivity, he taught them how to forge guns (illegal during this portion of Japan's history, and thus a significant advantage for White Beard) in order to overthrow the Emperor (then Emperor Go-Sai) and seize power. Hiro realizes that this is the thing he is here to fix, and convinces Kensei to destroy the cache of weapons.
As the three evacuate Yaeko's father, Hiro is forced to teleport himself and Yaeko out of the path of a bullet. This reveals his earlier deception to her, and the two kiss. Kensei, having tracked Hiro down out of concern, feels betrayed. Despite Hiro's insistence that it's a one-time occurrence, Kensei apparently returns to his previous ways and allies himself with White Beard, knocking Hiro out and delivering him to the conqueror in exchange for whatever he wants — he demands half the country and Yaeko as his princess. When Hiro claims that Kensei was supposed to be a hero, Kensei counters by saying that he will be once he conquers Japan, confidently stating that he will "change history."
After Yaeko helps to free Hiro, Kensei anticipates Hiro's first move to destroy White Beard's guns. Takezo confronts Hiro as he is spreading gunpowder over the area and attacks him, despite Hiro's pleas that they should work together. Hiro refuses to use his powers to win the duel, and they have a brief battle, during which Kensei knocks over a lamp and starts a fire. As the fire spreads, Hiro knocks Kensei's sword out of his hands, and offers the defeated Kensei a chance to escape. Kensei, torn apart by Hiro's betrayal and the loss of his love, claims that he will destroy everything Hiro holds dear (starting with Yaeko) and so Hiro leaves him to die. He finds Kensei's helmet and mask amidst the debris, and returns to Yaeko. She says that it is really he who is the legendary Takezo Kensei. However, Hiro knows that the legend isn't over yet: he must cut out his heart by leaving Yaeko behind. A tearful Yaeko promises to spread his tale. Hiro then leaves, thinking Kensei is dead and history restored.
 Revolutionary War
Due to his regenerative ability, Kensei not only survives the explosion, but is made immortal by the constant regeneration of his cells. At some point between 1671 and the founding of the Company, Takezo Kensei renames himself Adam Monroe, and becomes a British mercenary during the American Revolutionary War. It was not out of loyalty to the British, but simply to "escape boredom." He appears to think of himself as a god. At one point, Adam comes across a town in which the British regiment had been single-handedly slaughtered by a man named Evan, according to a survivor. Intrigued, Adam arrived at the scene to confront the man, and killed him.
However, after killing him, an entire army of clones rushes at the regiment and caused them to retreat, leaving Adam behind. He realizes that Evan is a human replicator and pursues him across a number of years, trying to kill the original Evan. Eventually, a number of Evan clones confront and taunt Adam, saying that in numbers there is strength and he is alone and can not possibly defeat them, and pushes Adam over a cliff into a river. It is at this point, defeated, Adam realized the need for followers.
 Modern times
Monroe creates the Company about three hundred years after his first encounter with Hiro Nakamura. As shown in "Truth & Consequences", when the Company began research into the Shanti virus, he attempted to release a highly virulent strain, claiming that something needs to be done about the endless cycle of destruction humanity seems to go through. Adam is locked away by Kaito Nakamura, never to be released again. Victoria Pratt mentions that in order to know as much as he did, he would have needed the help of some of the other founders; Robert Bishop, another founder, mentions that Maury Parkman and Daniel Linderman were among those who believed in his theories of their superiority over humanity.
As revealed in "Four Months Ago…", Adam escapes Company custody with the help of Peter Petrelli, a willing prisoner, by convincing him that the Company has no intention of helping him. Once free, Adam begins assassinating the Company's other founding members, using Maury Parkman as his agent. However, he is directly responsible for the death of Kaito Nakamura (as he promised that he would "lay waste to all that he [Hiro Nakamura] cared about").
In "Out of Time", Adam once again befriends a now amnesic Peter Petrelli, helping him to restore his memory through his absorbed regenerative abilities. Peter tells Adam about the effects of the virus in the future, and the two set out to destroy it, though Adam's true intentions remain to be seen. Adam leads Peter to Victoria Pratt, the creator of the immensely dangerous "Strain 138" that he attempted to release in 1977. Despite her lack of cooperation, Peter's telepathy allows him to retrieve the location of the virus. Adam unties Victoria, but kills her when she aims a shotgun at Peter's head, claiming to have been protecting Peter's life. As Peter walks out of the house, Adam looks at Victoria's lifeless body and says, "If anyone survives…I'll tell them I couldn't have done it without you". He dabs some of the blood from his healed chest wound, puts the Helix symbol on a picture of Victoria's face, and drops it on her corpse. The two then travel to Primatech Paper in Texas, where the virus is kept. Hiro intercepts them, freezing time and intending to kill Adam, but Peter remains unfrozen and refuses to let Hiro harm Adam.
 Buried alive
In the final episode of Volume 2, "Powerless", Adam and Peter make it to the vault where the virus is kept. While Adam goes to retrieve the virus, Peter is confronted by Nathan and Matt Parkman, who tell him that Adam is using him. Hiro teleports to the vault and confronts Adam. He claims that he "should have killed him long ago" before teleporting both of them away. When Ando asks Hiro if he killed Adam, Hiro responds that Adam can never harm anyone again. It is then shown that Adam has been buried alive in a Japanese cemetery near Kaito Nakamura screaming for "Carp" to let him out.
Series creator Tim Kring gave no indication that Adam could escape from his makeshift prison, saying "the fact that he can live forever makes this the most gruesome of internments." However, David Anders suggested that his character would be returning in future episodes. In the graphic novel The Ten Brides of Takezo Kensei, Monroe thinks that his new wife will find and rescue him from being buried alive.
In the third volume, Angela Petrelli has a precognitive dream in which Adam Monroe is seen alongside Maury Parkman, a woman that is either Tracy Strauss or her sister Barbara, Knox, and Sylar killing the Heroes.
In the final moments of "I Am Become Death", Hiro and Ando dig Adam up in hopes of gaining information on the formula. In the following episode, he is released from his casket prison under the pretense that he will assist Hiro and Ando in retrieving the formula. After arriving at a bar that he claims specializes in hiring superpowered individuals to commit crimes, he goads the bartender into attacking him, and lets Hiro take the blow instead. While Hiro lays unconscious and Ando goes to his aid, he escapes, only to be knocked unconscious himself by Knox in a back alley and taken away in a van on the orders of Arthur Petrelli.
Upon arriving with Knox, Monroe discovers that Arthur is indeed still alive, yet paralyzed. Realizing what Petrelli wants, a panicked Monroe tries to escape but is forced to touch Petrelli's hand. Petrelli then absorbs Adam's power of regeneration and heals himself while Adam appears to age centuries in mere seconds and crumbles to dust.
 Powers and abilities
Adam Monroe has the power of rapid cellular regeneration, allowing him to heal almost instantly from most injuries. He first displays it in "Lizards", where he is shot by three arrows. After the arrows are removed, his wounds heal and he revives. In "Kindred", he proceeds to repeatedly injure himself just to make sure it wasn't a trick played on him by Hiro. In the episode "Out of Time", he survives a massive gunpowder explosion. When his blood is injected into others, they are also healed.
In addition to allowing him to heal from any injury, Adam's power has effectively made him immortal. This is explained in the Season 2 episode "Cautionary Tales" by Angela Petrelli; his cells have regenerated so many times that they are now dying and regenerating in equilibrium, therefore halting his aging.