The Avengers represents a perfectly mixed team. There's ranged combat and brute muscle, super-science and sorcery, all of them working together toward a common end. There's also subtlety, and for those who forgot about her, it's likely the Black Widow is about to stab you in the back. No team should ever go into the field without a stealth and infiltration expert, and for those who think they can hold themselves up to the standard set by modern day kunoichi Natasha Romanoff (explanation for what a kunoichi is can be found here) it won't be an easy task. Fortunately the right build helps a great deal.
If you'd like to see character builds for the rest of the Avengers, as well as for Gotham City's vigilantes and the cast of Game of Thrones, check out the Character Conversions page on Improved Initiative!
Also, if you've ever wondered why so many superheroes wear colorful tights with their underwear on the outside here's the reason why!
Race and Attributes
As one of only two completely human members of the core Avengers team (Stark doesn't count, since he's a cyborg), human is the most versatile race for your Widow to start off with. The bonus feat and bonus skill point are going to come in handy in the field, and the race remains true to the character you're trying to recreate.
As far as her stats go the Black Widow needs to have both a high dexterity and a high charisma; these are going to be her two most important attributes. Intelligence grants more skill points, and strength is nice for additional damage. Neither constitution or wisdom should be her dump scores, as she'll need all the hit points she can get, and wisdom will be important for both resistance, as well as getting high results on important skill checks.
The Black Widow is a character who's seen and done a lot, and her traits should reflect this. Dangerously curious(+1 to use magic device, and use magic device is always a class skill for you) is great for duplicating her familiarity with S.H.I.E.L.D.'s scientific gadgets. Friends in high places (+1 to diplomacy and intimidate in an area where she's gathered information recently, and a +2 to diplomacy when dealing with government agents) can help represent the fact that she's an agent of higher powers through situational bonuses.Reactionary (+2 to initiative) is also a good choice, as going first might be the most useful weapon in your entire arsenal.
The ninja (Ultimate Combat 13) is what we'll be using as the basis for the Black Widow.
The ninja provides everything the Black Widow needs, and then some. This includes uncanny dodge and improved uncanny dodge, the ability to move without a trace, access to poison, and perhaps best of all a wide assortment of ninja tricks to help the ninja come out ahead. These tricks are powered by her ki pool, which is based on her charisma modifier.
Every ninja gains access to tricks, unusual powers that combine the supernatural manifestation of ki along with a lifetime of training and dedication. Some of them are more thematically appropriate than others for the Black Widow, though. Flurry of Stars, for instance, allows the player to throw 2 additional stars as part of a full attack with shuriken for the cost of a single ki point. All attacks are at a -2, but each attack gets full sneak attack dice if appropriate, and each shuriken can be poisoned; a solid representation of the Widow's Sting. The Rogue Talent trick is good for taking abilities like Trapspotter, which aren't on the ninja list. Practical abilities like Feather Fall, Darkvision, Sudden Disguise, Fast Stealth, and Vanishing Trick are all good picks as well. The obvious master trick (when the Widow reaches that high of a level) is Assassinate, but additional tricks like See The Unseen, Unarmed Combat Mastery, Master Disguise, or Ghost Step are all extremely useful abilities. All of these are located in Ultimate Combat in the Ninja class section.
What a Tangled Web We Weave...
The Black Widow is, quite frankly, a skill monkey. Just because players have a minimum of 9 skill points per level (8+1 for being human, assuming no intelligence modifier) though, that's no reason to not carefully think through which skills get selected.
Perception is an important skill for obvious reasons, as are stealth and bluff. Disguise is useful for passing as if you belong, and knowledge (local) is invaluable since the Widow has been everywhere at one time or another. Sleight of hand is a necessity for secreting weapons or stealing objects, and diplomacy is important for sweet talking your way into places. Linguistics is ideal for speaking multiple languages and deciphering written code, and intimidate can be a just the right tool when a social hammer is required. Disable device is a requirement for getting in and out without setting off alarms, and acrobatics is a necessity for moving around the battlefield without getting smashed.
Despite her great battlefield prowess, the Black Widow doesn't require as many feats as one would think.
What she does require is point blank shot (Core Rule Book 131), precise shot (Core Rule Book 131) andimproved unarmed strike (Core Rule Book 128). Most of the Widow's attacks are going to be ranged, and if she goes in somewhere with no weapons, she can still attack someone and get her full sneak attack damage as long as the opponent is flat-footed. For those who will be engaging in melee weapon finesse (Core Rule Book 136) is an essential feat as well, unless her strength is so close to her dexterity that it won't make a difference.
From this point onward feat selections becomes much more selective depending on what players want to do with their Widows. For poisoners adder strike (Ultimate Combat 89) allows poison to be applied to unarmed strikes. For flatterers and diplomacy experts, betrayer (Ultimate Combat 89) is useful for getting in close to someone, and then stabbing them when they least expect it. If your DM is using the massive damage rules, or is fond of creatures with damage reduction, then clustered shots (Ultimate Combat 92) might be a good thing to have on hand. Skill focus (Core Rule Book 134) is a good choice for making sure that your Widow has an unparalleled skill or skills, depending on how many times you take that feat.
The Black Widow is a spy and assassin, and as such she tends to have a number of the tools of her trade on her person. Masterwork thieves tools, disguise kits, and other such things are par for the course, but this section will cover some things that you might not have thought to put in your pack.
First off, alchemical items. Tanglefoot bags trip up the bad guys even if they make their saves, thunderstones can be a great surprise weapon as well as a marvelous distraction, antitoxin and antiplague can really beef up your saves against nasty poisons and diseases, and troll styptic makes for an ideal field dressing when there's no cleric on hand (or the cleric is down). Weapon blanches are good choices for overcoming damage reductions, and smokesticks can allow you a clean get away. All these items may be found in the Core Rule Book, Seekers of Secrets, and Ultimate Equipment. In addition to these purely alchemical items it's a good idea to make sure the Widow has an ideal selection of mundane gear for getting along on an adventure. Everything from a bandolier to the right tool sets can make the difference, so don't get caught up in how shiny magical gear can be.
As far as the Widow's magical equipment, there are a lot of possibilities. Stat boosting items are always a plus, with dexterity being top of the list. Boots that increase stealth, and rings that increase armor or saves are equally good investments since these things can never be too high. Specific items like the Gloves of Reconnaissance (Ultimate Equipment) are an invaluable tool for any spy, as it lets them "see" through solid material. Like doors. Or chests that may or may not be trapped.
As far as her weapons go the choices depend a great deal on the player. Those who multiclass and gain the use of a bow might find it's useful to use magical arrows to pepper the enemy with enchanted arrows purchased specifically for dealing with them (bane, elemental damage, holy, etc.). The same strategy works with shuriken that have been properly poisoned, or which have certain magical properties that makes them deadly against certain types of foes. Unarmed strikes are always an option, but the ninja also has access to eastern weapons like the katana that, when enhanced magically and envenomed just to be safe, can be combat enders.