Kurogane

Kurogane tells the tale of "Hitokiri Jintetsu", a boy just in his teens who has become renowned for his swordsmanship and his proficiency in killing, despite his young age. After getting revenge on his father's murderer, Jintetsu is forced to constantly flee from bounty hunters who wish to collect the price on his head. Despite the skill of these killers, none are capable of besting him in combat, and he's consistently victorious. The deaths that lay in wake earn him the "Hitokiri" moniker. Unfortunately, as fate would have it, he ends up meeting his fate not by someone of equal or even greater skill, but rather by a pack of wild dogs. Though he eventually slays them all, he is fatally wounded.

A ronin named Genkichi finds him in his final moments, and he tells the man that he is ready to die so he should hurry up and finish him off. Genkichi informs him that he has no intention of killing him since his wounds will take care of that anyway. When Jintetsu finally dies of blood loss, the former samurai brings him back to his home and actually manages to revive him. However, there is a horrible catch. A great portion of the boy's body was destroyed, so in order to compensate, Genkichi constructed a partially mechanized body of steel and wood (though there is an inconsistency where the first chapter of manga shows him with a fully mechanical body, while the end of the manga portrays him as only be partly mechanical). Although he also managed to repair most of the killer's nerves and motor functions, he is unable to repair his vocal cords and right eye, slightly impairing the swordsman's abilities and making him mute. Jintetsu does not take to his revival kindly, but is forced into inaction since he isn't used to his body just yet. He is then given new clothes to hide his appearance, and a new sword that Genkichi also notes as being mechanized.

Eventually, Jintetsu comes to accept his existence, though he later finds out Genkichi only built him to prove to his former master his ingenuity. His creator then dies after getting his revenge on the said master, apologizing to the assassin for using him before succumbing to his wounds. Jintetsu, surprisingly, seems to forgive him, burying him rather than leaving his corpse as it lay. He then uses the sword he had given him as a grave marker, and begins to walk away from the scene. However, before he can leave the area, the sword speaks and implores Jintetsu to not leave his equipment behind and to take him with him. Though surprised, he obliges, and the sword reveals his name to be Haganemaru, and that he can read Jintetsu's thoughts since the two share a mind. Therefore, not only is he his weapon, but can also speak for him on his behalf. The manga then follows their adventures as Jintetsu works as an assassin-for-hire and Haganemaru as his instrument of death. All the while he is trying to throw away his past, and escape from the various people who are seeking vengeance for what he had done. Along the way he meets a particularly determined young girl named Makoto, who wants to avenge the destruction of her family, and subsequently ends up becoming a makeshift companion. He also earns the nickname of "Jintetsu of Steel" for his metallic face, which many assume is a mask despite it actually being an irremovable part of his body since his revival.

The manga ends prematurely after Jintetsu has a confrontation with a swordswoman who can match him blade for blade. He leaves for parts unknown afterward (as always), with Makoto in close pursuit.

Not to be confused with Kurogane no Linebarrels.

Tropes used in Kurogane include:

Anachronism Stew - It is the Edo period. Jintetsu is basically a cyborg.
Big Damn Heroes - Jintetsu.
Blade Below the Shoulder - Jintetsu uses a small knife in his mechanical elbow as a sneak attack, but oddly, only once or twice.
Desperately Looking for a Purpose In Life - Makoto chooses revenge as her purpose, even though she realizes that deep down, Jintetsu didn't ruin her family. At the end of the manga, it's implied she's found a new purpose in repaying Jintetsu for always saving her.
The Drifter - Jintetsu. And nearly any other swordsman in the series. It's actually their official occupation!
Dynamic Entry - Jin's apparent rules of engagement: 1) Always arrive Just in Time, 2) Always be accompanied by a murder of crows, and 3) Just appearing out of nowere is as effective of blocking incoming blades.
Dying Alone - Jin predicts this to be his future fate.
Empathic Weapon - Haganemaru can read Jintetsu's thoughts at a short distance and speaks for him. The sword, as it turns out, is also a Equippable Ally, as it was Genkichi's first pass at reviving people through technology.
Empty Shell - Jintetsu refers to himself as this, and states that's what he sees in Koyoi as well. The reality is quite different, but he likes to be hard on himself.
Honor Before Reason - Makoto believes Jintetsu destroyed her family and thus completely ruined her life. However, instead of taking revenge on him and killing him while he sleeps, she opts to only kill him when she can beat him in outright combat. Yet, she knows he can kick her ass.
Hot Chick with a Sword - Koyoi.
I Just Want to Be Normal - "I wanted ordinary happiness that came from living a simple life in a simple village. I refuse to forget that. When I do, I'll end up like you."
Jidai Geki
Just in Time - Jintetsu doesn't understand the concept of arriving at any other time.
Kid Hero
Last Request - Renji asks Jintetsu to tell Makoto that he was the one who ruined her family all along. In order to preserve her memory of him, Jintetsu does not oblige this request.
Master Swordsman - Jintetsu, despite being Just a Kid.
Only Known by Their Nickname - Assassins are rarely known by just their name, being called their full "designation" by everyone except their friends. If they have any. Examples: Jintetsu is known as "Jintetsu of Steel," Renji is called "Renji the Firewalker," and Makoto is called "Makoto the Scarlet Sparrow."
Roaring Rampage of Revenge - The impetus for the series, at least in part, but also the basis of a handful of the other events in it.
Talking Weapon - Haganemaru.
That Man Is Dead - In a tear jerking scene, Jintetsu finally sees his childhood friend again, one of the few bright spots of his past. She desperately wants him to stay with her, and promises she won't ask where he's been or what he's done. He sadly responds that "Jintetsu is dead," and that she had only seen "his ghost."
The Only One Allowed to Defeat You - Makoto's frequently invokes this when she suddenly finds herself helping Jintetsu.
Unknown Rival - Makoto. Jintetsu infuriates her on purpose though, as she's easier to keep an eye on when she's relentlessly pursuing him.
Wholesome Crossdresser - Makoto. Jintetsu and other women see through her disguise though, with the women complaining that she's "wasting her good looks."
You Remind Me of X - "Why did you save my life?" "You reminded me of me... when I was young."
Zero-Approval Gambit - Jintetsu is all about sacrificing himself for the sake of others. However, since dying doesn't come easy for someone so badass, sacrificing himself usually involves taking on grave injuries and perpetuating the popular belief that "Jintetsu of Steel" is an enormous dick.