Angela Ziegler was a renowned scientist, doctor, and pacifist who believed in peace and healing, not in war. While Overwatch was clearly a militaristic organization, Ziegler managed to find a way to work with them, while still getting what she wanted. A guardian angel to some, and an angel of mercy to others, she saved countless lives during her time with the organization, and hasn't let its dissolution stop her from doing what she feels is right.
For those who've been trying to figure out a way to translate Mercy to the Pathfinder RPG, this guide will offer some starting points. While the mechanics may be different in many places, the spirit of the character should shine through quite brightly in some places.
For other character conversions, from the Hulk to the Dread Pirate Roberts, head over to Improved Initiative's Character Conversions page.
The Makings Of A Hero
Angela is human, as far as we can tell. It's also a good race to pick going forward. She'll need a high Charisma and Wisdom, and while good physical attributes are nice to have, they are not a priority for Mercy.
As to her traits, there are a number to choose from. Devoted Healer (whenever you take 20 to treat deadly wounds, heal an additional 1d4 hit points to those under your care) is perhaps the most appropriate. However, Heavenly Touch (you can touch a dying target to stabilize them as a standard action), Community-Minded (any morale bonus you confer onto your allies with your own spells or abilities lasts an additional 2 rounds), or Dangerously Curious (gain a +1 trait bonus on Use Magic Device checks, and Use Magic Device becomes a class skill for you) are also good options to consider.
Mercy is a consummate healer, and a brilliant scientist, but she is also deeply devoted to her cause. Fiercely loyal, and with a strict sense of right and wrong, the most fitting class to put her in is the paladin. However, it's best to start her as a warrior of the holy light.
This archetype gives you extra lay on hands per day, and allows you to create an aura that provides morale bonuses to your allies, in addition to healing them of ability damage, and at higher levels it provides additional protection while acting as actual daylight. You lose all spellcasting levels, though, and will not be able to cast any spells. It also means there's no need to pray each day, allowing Mercy to simply be carried by the righteousness of her beliefs, and her cause.
This should give you plenty of options when it comes to keeping your allies on their feet, and fighting against even impossible odds.
Skills and Feats
When it comes to skills, you need to be careful with your expenditures. Even that extra human skill point will only go so far. The basic layout you'll want is Heal, Use Magic Device, Fly, and either Craft (Alchemy) if you want to embrace her scientist aspect, or Knowledge (Arcana) if you want to take the "science is magic here" approach.
As to Mercy's feats, there are a lot to choose from. The two you'll want to start with are Point Blank Shot and Precise Shot, followed by Channel Ray. This allows you to turn your channel energy into a ranged touch attack, which can be perfect for helping out allies when you don't have the ability to close the distance.
You'll also want to take Greater Mercy (when you use lay on hands on a target with no conditions your mercy can remove, instead heal another d6 points of damage), and when you meet the prerequisites, Ultimate Mercy (expend 10 uses of lay on hands to bring a dead ally back to life as raise dead, and may accept 1 temporary negative level instead of paying the material component cost). That's a pricey ability to use, though, even with the boost from warrior of the holy light, so it might be a good idea to take Extra Lay on Hands a time or two.
Additional feats you may wish to consider include Selective Channel (which eliminates certain squares from getting the benefits of your channeled energy), Steelskin Channel (which grants DR 2/- to those healed by your channeled energy for a limited time), and Channeled Revival (which allows you to expend 3 uses of your channeled energy in order to cast breath of life as a full-round action).
Story and Gear
One of the most immediately noticeable things about Mercy is her valkyrie suit. If you want to realize this item through magical means, then one of the easiest ways to do so is via celestial armor (light or medium, whichever you prefer). Alternative methods include playing Mercy as an aasimar in order to gain natural wings, or investing in a technological item like a jet pack, but those alternatives can take a great deal more time and effort to acquire.
As to Mercy's Caduceus Staff and her Caduceus Blaster, they're fairly easy to mimic assuming you have access to the proper magic and technology. A staff of healing is ideal for most adventurers, or you could use a staff of life if you have access to items that cost over 100,000 gold pieces. The blaster could be easily replicated by an arc pistol, but that is both an expensive piece of technology, and something that will require you to burn a feat slot to use if they aren't very common in your setting.
Lastly, given Mercy's predilection for gliding, a ring of feather fall is definitely a no-brainer when it comes to purchases.
The last thing you have to decide is what story you want to tell with her. Is your Mercy a child of the crusades, trying to heal those who've been left behind? Are you a dedicated scientist, trying to understand the powers of the universe through the lens of magical laws and alchemical rules? Or are you seeking something that can only be found on the fringes of the world, which requires you to travel with companions?
The choice, as always, is up to you.