There is a shadow that lurks over the world's battlefields, leaving the dried husks of its victims in its wake. A creature whose face is so terrifying there are those who believe it could never belong to a man. It does, though. Gabriel Reyes, once a proud graduate of the U.S. Soldier Enhancement Program, and the head of Blackwatch, has become something inhuman. A force of death and murder known to most only as Reaper. If you've wanted to bring a character like Reaper to your Pathfinder table, this guide should help get you started.
If you're looking for other Overwatch guides (such as this guide for building Mercy), or for characters like the Avengers, the cast of Game of Thrones, or even Batman's Rogues Gallery, check out the Character Conversion page on Improved Initiative.
Gabriel Reyes was, at least in the beginning, human. That gives you an extra feat, and an extra skill point every level. You're going to want to focus on dexterity as your chief stat, though constitution and strength will also be useful. Charisma will also be useful, and should receive either a middling or high score.
As to Reyes's traits, well, there are a lot that are fitting. Bloodthirsty (gain a +1 trait bonus on damage that reduces a creature to zero hit points, and on confirmed critical hits) fits with his nature, but Never Stop Shooting (act as if you have the diehard feat when reduced to zero or fewer hit points, but you may only draw, reload, or fire a gun) is also useful. For his second trait, though, one of the most fitting is Blighted Physiology (+1 natural armor bonus, but become sickened for one round anytime you receive magical healing). However, if you wanted to focus more on his role as a trainer and commander of a black ops team, then Aspiring Hellknight (+1 trait bonus to intimidate, and intimidate is always a class skill) could also work.
Reyes was a dangerous soldier, and a high-functioning sociopath before the genetic tampering that turned him into the masked terrorist known as Reaper. He was an expert infiltrator, a remorseless killer, and a top operative. Which is why one of the most ideal choices for Reaper's base is the ninja.
Ninjas gain sneak attack, proficiency with a variety of weapons, and a ki pool that lets them increase their stealth checks, make an extra attack as part of a full-attack action, or increase their speed. This ki pool is equal to half their ninja level, plus their charisma modifier, which is why a decent charisma score is important. Ninjas also gain Uncanny Dodge, and Improved Uncanny Dodge, No Trace (which allows them to evade being tracked, and makes it hard to locate them if they stay still), and Ninja Tricks.
Those tricks are where a lot of your capabilities are going to come from. The first one you'll want to take is rogue talent (firearm training) so Reaper is fully proficient with firearms. Shadow Clone is also useful, especially if you find yourself in a cross fire. Vanishing Trick is good for a fast getaway, and fast stealth lets you flit from shadow to shadow without taking a hit to your stealth check. And if you take enough levels of ninja to get master tricks, then Ghost Step will allow you to step through barriers as if you were incorporeal.
In addition to ninja, you'll want to take levels of Shadowdancer. This gives you the infamous shadow jump, in addition to a slew of other powers. You can take this prestige class fairly early, as the prerequisites aren't very high, but it's important to make sure you have enough sneak attack to satisfy you, as that's where most of your damage will come from. However, Shadowdancer and Ninja will stack for the purposes of your improved uncanny dodge, and you will get some additional rogue talents as you progress in level.
Reaper is a master of infiltration, and he cuts a bloody swath across any battlefield he crosses. Ninjas and shadowdancers both get a fairly large number of skill points, as well, which can come in handy. You'll want to have Stealth, Intimidation, Perception, Acrobatics, Bluff, Sense Motive, and Use Magic Device.
As to feats, the three you're going to need are Dodge, Mobility, and Combat Reflexes, as they qualify you for Shadowdancer. In addition to these feats, you'll want to take Point Blank Shot, and Precise Shot.
You should also invest in Two-Weapon Fighting, Improved Two-Weapon Fighting, and if your campaign goes on long enough, Greater Two-Weapon Fighting. You may also want to invest in Quick Draw, in the event you choose to draw new weapons when your old ones run out of ammo rather than taking the time to reload mid-fight.
Perhaps the most useful item for Reaper is the Gunman's Duster. It gives you a +4 armor bonus, and a +2 luck bonus to touch AC against enemy firearm attacks. Another useful item for him to have is the Endless Bandolier, as it will keep a number of spare guns and ammo in an extradimensional space, ready to draw and fire at a moment's notice. Even better, you can Quick Draw them! Unfortunately, wielding a two-handed firearm in one hand is problematic with Pathfinder's rules, so you'd be much better off dual-wielding dragon pistols. As long as they're filled with slugs, you can get your sneak attack damage on your shots. However, a standard spread with tanglefoot goo in it can debuff your enemies with one hand, and then you can plug them with the other. You might even be able to ask for a double-barrel version, if your DM is generous.
You should also seek out a Penumbra Tattoo in order to protect you from direct sunlight.
While all of the abilities listed up to this point are useful, there's a little extra oomph Reaper needs. Something to reflect the strange nature of the transformation his body went through, and the state of living death he finds himself in.
That's why you should add the vampirism corruption.
While the character doesn't drink blood like a classic vampire, he does siphon the life energy away from his fallen foes. Additionally, his ability to heal at an accelerated rate, and to become little more than an incorporeal cloud, aren't really brought across through the abilities listed above. Which is why you should acquire, at minimum, Vampiric Grace (which gives you Dodge as a bonus feat, allowing you to meet your Shadowdancer prerequisites with fewer resources), Unlife (which gives you fast healing one for a number of minutes per day), and Greater Unlife (which gives you fast healing three for a number of rounds per day), and True Unlife (immediately take on gaseous form and remain conscious if brought below zero hit points, activating your fast healing if you have minutes remaining for the day).
You don't have to take the corruption at level one, but it helps if you want to pump up your power asap.