Mysterious Mercenary Pursuer

This is a character with the following traits:

A mercenary, bounty hunter or assassin.
Antagonist sent to capture/kill protagonist(s)
Relentless in pursuit of protagonists. Usually a Determinator, often an Implacable Man. Sometimes the protagonists will try to pay him off, but he'll always ignore the offer and get killed off.
Mysterious in one or more gimmicky ways. Sometimes he's The Faceless, sometimes The Voiceless, sometimes he isn't named except in an Everyone Calls Him "Barkeep" way, sometimes he wears a concealing suit of armor, sometimes he has an unexplained facial scar, sometimes he turns out be a she...

The difficulties involved in getting such incredible commitment from Hired Guns may or may not be addressed. Often a Hero-Killer.

Examples of Mysterious Mercenary Pursuer include:

Comic Book

Toyota the ninja from Y: The Last Man, with the twist that she eventually gets more characterization and rather than a stoic, silent ninja, turns out to be a huge bitch and Arrogant Ninjitsu Chick.
The Man in Black from the Farscape comics, who is The Faceless and The Voiceless for the first few story arcs. He's trying to kill Deke, Crichton and Aeryn's newborn son.

Film

Boba Fett from Star Wars. He talks very little and wears a concealing suit of armor. Was mysterious in the original films, but dozens of novels and the prequel trilogy killed much of his mystique.
- Princess Leia was able to pose as one named Boushh at the start of Return of the Jedi.
The unnamed Psycho Ex-Girlfriend from The Blues Brothers played by Carrie Fisher qualifies in method, if not in profession.
Angel Eyes from The Good, The Bad & The Ugly. "Trouble is when I'm paid, I always see my job through..."
- It's not the protagonists he pursues, it's poor Bill Carson. But other than that...
Leonard Smalls, the Lone Biker of the Apocalypse, from Raising Arizona.
The Coen brothers seem to like stories with guys like this. My first thought was Anton from No Country for Old Men.
Joe Lefors in Butch Cassidy and the Sundance Kid. So mysterious, he's never even properly seen in the film. Butch and Sundance see him once from a distance of a mile or more, then later on see someone they assume is him from behind, although it's not clear if it's really Lefors or just paranoia on their part.
- You can also tell it's Joe Lefors because he always wears a white straw hat.
The assassin sent to get Carter in Get Carter. He has no dialogue and is distinguished only by his signet ring, inscribed with a J.
Dragan, the assassin hired by Serbian mobsters in Layer Cake. Seen only briefly at the end, he tracks the protagonist through almost the entire film and is always a step ahead.
Mr. Shush, a taciturn hitman sent to kill Jimmy the Saint's gang in Things to Do in Denver When You're Dead.
The Man in Black was this to the three bandits who kidnapped Buttercup in The Princess Bride.

Literature

The unnamed magic-resistant mercenary in The Bartimaeus Trilogy.
- His name is revealed in the final book, although only mentioned once: Verroq
Pie'O'Pah of Imajica initially seems like one of these, but turns out to be more complex. He winds up as the Love Interest.
Robert Aspirin's 6th Myth series book Little Myth Marker has Skeeve being hunted by a mysterious assassin nobody (except victims) has ever seen named The Ax.

Live Action TV

The Operative and the Hands of Blue in Firefly/Serenity.
Keamy in Lost
The Cousins in Breaking Bad are two sharkskin-suited professional killers who wield a deadly axe and target the main character for assassination until his boss, who is their associate, intervenes.

Machinima

The Leet World's Domination Guy. Gives up the pursuit, and even helps the protagonists, after a Defeat Means Friendship / Worthy Opponent moment with Cortez. Is eventually killed by his employer for his trouble.
Wyoming from Red vs. Blue.

Video Games

Inverted in Mirrors Edge: The "white guy" is a mercenary assassin, is both faceless and voiceless and wears the signature white suit and mask and turns out to be a woman. However he's trying to get away from Faith who is quite the determinator in pursuit and doesn't get the hint to leave things be.
Kanon in Wild ARMs 2 is introduced this way.
- Chelle in Wild ARMs XF' begins this way, but her motives are revealed as the game goes on.
The Jackal from Illusion of Gaia" chases you through of the game, but only shows up once near the end.
- Zagi from Tales of Vesperia chases you though out the entire game, gets stronger every time, and will not die!!!!!.
Pison in The 7th Saga. He comes back (twice!) after you beat him the first time, each time getting tougher, and with more special attacks.

Webcomic

Sluggy Freelance's Nash is a good example. He even survived a very lethal-looking wound.
Miko is introduced this way in Order of the Stick.
Mr. Blank from Sam and Fuzzy... Until we learn the real reason he's been chasing Sam following Sam being blamed for killing the Ninja Mafia leadership.

Western Animation

Combustion Man from Avatar: The Last Airbender. He's The Voiceless, he has a different form of Fire Bending, a strange tattoo on his head, we never find out his real name...
- Sparky sparky boom man!
A Clint Eastwood Expy hunted the Gummi Bears.
In the Aladdin series the Muhktar is first introduced as this, a mysterious, caped figure relentlessly hunting Genie.
Lockdown of Transformers Animated fame, though he's more talkative than most. What can he say, he loves his work.

Real Life

Xanthippus. He was a mercenary Spartan general hired by the Carthaginians to fight Rome in the First Punic War. He trained their soldiers, corrected flaws in their battle strategy, and routed the Romans while capturing their consul. It was apparently the only battle he fought before sailing home to Lacedaemon.
- Considering the result of the war he might well have been relieved to get home.