The Leader of the Gung Ho Guns and the right hand man of Knives.
Although the anime lacks a backstory for Legato, the manga provides a backstory for how the sociopathic killer came to be. Legato was born as a slave and sold repeatedly as a sex slave at a very young age. He met Knives when Knives killed all the residents of a town he was in as well as the group of men who were molesting him. Legato immediately saw Knives as a god of sorts and pledged a loyalty to him. Knives saw him as a mistake when killing as Legato begged to let him serve Knives. Knives then sees that Legato survived through his own powers that he was unaware of. Knives is also the man who gave Legato his name since as a slave he lacked one.
The anime does not go over these details but tells a different story. Some time before the main plot of the story began, Legato met with Knives and saw him as his god. Knives gave Legato Vash's left arm that would grant him some of the powers the two brothers possessed. Since Knives was still in a recovery state after Vash's attack, he orders Legato to cause Vash nothing but suffering on the planet but never asks that he kill his brother. Legato honors this request as he creates the team of merciless killers known as the Gung Ho Guns.
Both anime and manga counterparts of Legato share the same viewpoint on humanity in that they are totally useless.
Legato was created by Yasuhiro Nightow who is the original mangaka for all of Trigun. The anime version of Legato was created by Satoshi Nishimura of Madhouse Studios.
In the anime, Legato makes his first appearence in episode 12 in the anime. When Legato encounter Vash for the first time, the two do not physically talk but instead use telepathic powers to communicate. Legato informs Vash that he works for Knives and that his duty is to cause him eternal suffering in the world. Legato tricks Vash into getting worked up over a small child before leaving Vash a "gift": the severed head of a shoe maker Vash encountered earlier.
Legato wouldn't appear again until episode 15 where he would encounter and eventually lay waste to a group called the Rodrick Gang. This would also mark the first appearence for the Gung Ho Guns as a whole. He allowed a different Gung Ho Gun to try to take out Vash but as each one failed, he killed them as a result (other were wise to kill themselves to avoid his wrath). After Vash's staged disappearence, Legato remained on the look out for Vash while continuing to fuel his own vendetta against the world. In a later episode, he would cause Chapel the Evergreen to seriously wound Nicholas Wolfwood and cause his death later on in the episode.
The final battle between Vash and Legato would happen in episode 24 after Vash has dispatched the last Gung Ho Gun and demands Legato to reveal where Knives is. Legato refuses at first and instead uses his powers to control the entire population of a nearby town to capture Milly and Meryl and kill them in front of Vash. Legato explains that in order to save the girls and the people, Vash would have to kill him. Vash is conflicted about doing this but eventually kills Legato to end his manipulation of the people and save the girls. Through this, and true to his word, Legato would cause Vash suffering through his death as Legato would be the first and only person Vash killed willingly.
In the manga, Legato and Vash meet under identical circumstances that they did in the anime. However, Legato gives Vash a coin box with broken coins in it with a game: collect all the other halves of the coins. He explained that each of the Gung Ho Guns would carry the other pieces and that if he were to collect all the pieces, something interesting would happen. Vash tries to avoid playing this game but Legato pushes him at every opportunity to participate. Thus when Knives is resurrected, he crushes Legato's body to punish him for using his powers for his own personal uses as opposed to what Knives told him to do.
As a result of this punishment, Legato is restrained to a metal coffin carried around by a large Orge. Even though his body was crippled immensely, he still demonstrates he has his powers by torturing a group of marines and even Midvalley at one point. As time goes on, he switches coffins and finds that he can manipulate a mechanical body using various invisible threads as well as his psychic abilities. With this new body comes a new weapon in the form of a ball and a chain that he uses as a projectile.
In his final encounter with Vash, he does his best to kill Vash but when Vash places the last coin into place in the coin box, Legato's powers wane away as Vash goes in to kill him in a final battle.
In both the manga and the anime, Legato has the ability to use psychic powers to manipulate people. He obtained these powers from Knives who gave Legato Vash's left arm, which was endowed with some of the god-like powers the brothers possess. When he does this, it is usually to make a person or a group of people do something against their will. Examples of this include when he made a group of men from the Roderick Gang kill one another, making an entire population of a town disappear, controlling Chapel the Evergreen to kill Wolfwood, and cramming a group of soldiers into a tight truck by manipulating their bodies.
In the manga, Legato is also said to have the power to control threads, which are invisible barriers that can control things as well. The biggest example of when he used this was with his first encounter with Knives as a child. He used this power to survive Knives' attack and impressed the killer to allow him to be his right hand man.