Pietro Maximoff and his twin sister were offered asylum by Magneto when their mutant powers manifested. Taking the codename Quicksilver, Pietro has become one of the Brotherhood's most zealous soldiers, if not always its most clever or capable. For nearly a decade, he has done whatever the Brotherhood's mysterious leader has required of him, progressively becoming more violent even as his ability to travel faster than the speed of sound has grown more powerful. He has become one of the most recognizable figures of The Brotherhood, and his acts of sabotage and terrorism have made him a person of interest to most of the world's law enforcement organizations.
Pietro Maximoff never knew his mother. She died shortly after she gave birth to him and his twin sister Wanda. Their mother had found shelter near the Wundagore Mountain, and the newborn twins were left in the care of a midwife. This midwife eventually gave the infants to Django and Marya Maximoff, a young Romani couple who had recently lost children of their own. The couple named the children and raised them like all the other children in the tribe. The twins grew up very poor, but fit in well with the nomadic tribe. Nobody ever bothered to tell them that they had been adopted, and the children were most likely better off for it.
One could almost argue that Pietro and Wanda had ideal lives despite their poverty. They got to see the majority of Europe, and had a loving family. But then Wanda's mutant ability to cause bad luck activated for the first time. She caused a couple of serious fires in the village the tribe had camped near. This riled up the locals, and they were prepared to kill the young girl for being a suspected witch, and kill her brother because he was probably guilty of something. This did not sit right with Pietro, whose mutant power of superspeed picked that exact moment to kick in. Pietro was able to rescue Wanda, and from that moment on he had a distrust and hatred of other people, along with an almost pathological desire to keep Wanda safe.
Unbeknownst to the twins, they had been observed by a powerful mutant named Magneto. He offered them protection from the people who were hunting them. But even more importantly, he offered them answers. They weren't freaks, or possessed by demons. They were gods among ants. People were always going to fear and hate them no matter what they did. But they didn't have to take the abuse. They could join him in his crusade to make the world a safer place for mutants. Pietro suspended any disbelief he might have had and joined Magneto's Brotherhood.
Pietro and Wanda became Quicksilver and the Scarlet Witch, and began training with Magneto's fledgling group of "evil" mutants. Pietro found himself very quickly learning to outrun sound waves, and continued to get faster from there. Wanda benefitted even more from the training, learning a degree of control over her erratic abilities. Soon they were out in the field, rescuing mutant prisoners, destroying anti-mutant research facilities, and occasionally making targets disappear. Working covertly was of paramount importance, while the Brotherhood conducted recruiting operations. Quicksilver proved to have a knack for showing up at just the right time in order to save a poor oppressed mutant. This made him a vital part of swelling the Brotherhood's ranks.
Quicksilver trained pretty hard, working for the Brotherhood with a sort of religious zeal. Wanda, however, wasn't quite so committed. However, their mutual talents quickly became apparent, and the two were gradually advanced to leadership roles within the organization. However, despite serving faithfully for a decade, the twins never had direct contact with their Leader. Quicksilver was prepared to accept this, due to his blind faith in the organization and its goals. A major source of contention between he and Wanda began to develop over the amount of trust they ought to have in The Brotherhood and its aims.
The Brotherhood was becoming a big deal, and a legitimate threat to the security of governments everywhere. This forced the operatives to work in small, scattered cells. This led to Quicksilver being on his own more frequently, commanding his small unit without Wanda's steadying influence. Gradually, his attacks became a bit more bold, and a bit more public. So much so that he is now seen as one of the most public faces of The Brotherhood. His orders have progressively become more questionable, and his actions have become more violent, making Quicksilver a person of interest to virtually every law enforcement agency in the world despite his short and so far unimpressive career.
Quicksilver's brain has to keep up with his body when moving at accelerated speeds, so his mental processing speeds get progressively faster the more quickly he runs. This doesn't make him smarter, per se, as all of his extra mental aptitude basically goes toward maintaining his speed and avoiding obstacles. It just means he thinks as fast as he moves. Quicksilver is able to think at rates relative to his physical speed. If he's resting, his mental processes are only slightly faster than a normal person's. When moving at hypersonic speeds, he calculates information way faster than a supercomputer. In practical terms, this means he is able to do things like read through a book simply by flipping the pages like a flip book. However, his retention of information he acquires this way is basically nonexistant. For example: Quicksilver could flip through the pages of a French textbook in a fraction of a second, and then run to France and order a baguette with perfect grammar. However, by the time the waitress brought him his baguette he wouldn't even remember enough French to ask for some extra beurre. For another example: he could quickly glance at the floorplan for a highly fortified prison, find the cell in which his target is located, and then zoom inside the building right in front of the cell. But by the time he got the cell door open, he might have difficulty remembering which exit route he had selected. In order to actually learn a skill, Quicksilver has to do it the long hard way. He rarely does this.
Quicksilver's body is specifically optimized for moving at high speeds. He metabolizes virtually all of the food he consumes (quite a feat, since he eats a lot). His structure is more durable than a baseline human's, allowing him to hold up under all the different pressures and frictions associated with hypersonic travel. He has superhuman stamina, allowing him to run across entire oceans at ridiculous speeds without being worn out at the end of his trip. He can breathe without difficulty, even when moving at his fastest speeds. Also, his eyeballs don't explode out of his head when he's running.
Quicksilver's mutant powers enable him to run at speeds around Mach 5.
Quicksilver is a high ranking member of The Brotherhood, in charge of a small cell. The group recieves some amount of funding, but offers greater rewards to those who prove resourceful on their own. Because of this, Quicksilver's unit is used to making do with less, often operating with equipment that seems embarrassing when compared to that of more prestigious groups. However, through this unit he is connected to quite a few powerful mutants, which balances out the equation considerably. Using the resources at his disposable, he is able to conduct individual or small unit attacks virtually anywhere in the world with little to no advance notice.
Quicksilver analyzes things quickly, and then acts immediately. It's his greatest boon as a leader of Evil Mutants. His decisiveness often makes up for his shoddy plans and his multiple other failings as a leader.
Ever since birth, Pietro has been virtually inseperable from his twin sister Wanda. She is easily the most important person in the world to him, even well into his twenties. Pietro sees himself as Wanda's protector, and often treats her more like a little sister than a twin. However, it is really Pietro who is dependent upon Wanda, primarily because she is one of the only people capable of talking him down when he's about to do something incredibly stupid. She frequently plays the "Good Cop" to his "Bad And Somewhat Douchey Cop." Pietro and Wanda don't know this, but although their mother is dead, their father is still very much alive. It's also possible they have a sibling or two out there somewhere. Whether they will ever experience a happy family reunion is unclear at this point.
Quicksilver is fluent in the Romani dialect of his adopted family, knows enough phrases to get by in a few languages, and speaks mostly fluent English. He has worked hard to remove the traces of accent from his voice when speaking English, as most of his operations occur there. However, the road to sounding like a native speaker has not been easy, and he has only been partially successful. Still, given the melting pot culture of America, he doesn't stand out too terribly as long as he doesn't try to give a speech or something.
Quicksilver and his sister were taken in by a tribe of Romani as infants, and spent their formative years travelling all over Europe. The twins' first language was the dialect of their particular Romani tribe, but they also speak bastardized versions of the official languages of the different European countries they've lived in. Enough to get by, anyway. His early impressions of what is morally and socially acceptable were shaped by traditional Romani beliefs. This makes him something of an outsider among people in the US and most of Europe, although he usually adapts well to the practices of those around him. He is accustomed to surviving with very little, and has lived as a nomad for almost all of his life. He is capable of living in a relatively antiquated manner, though his family didn't use the stereotypical horse-drawn caravan. He is also familiar with the music, traditional stories, and forms of entertainment used by his adopted family. Although the modern day Romanis have many different dialects and are a very diverse people, he would most likely be able to find common ground with them in whatever part of the world he might find them.
Because Quicksilver has lived virtually everywhere in Europe for at least a few days, and because the Romani tribe to which he belonged owned no property, Quicksilver doesn't owe allegiance to any nation. He considers every country in the world equally crappy. This makes him a bit of a social chameleon, always apart but never truly alien. It also gives him a broader perspective than most people have.
Chalk it up to gullibility, a defect in character, or whatever. But Pietro is pretty easy to manipulate. His motivations are pretty simple, and anyone who can make big enough promises (and back them up) will probably be able to get Pietro to do what they want. His morality is a constantly shifting thing, and he will probably always need someone to tell him what to think. He sees himself quite differently though, as a paragon of personal strength.
Do a quick search on Urban Dictionary if you're unfamiliar term. Name a bad stereotype about Europeans that's propagated by American media, and Quicksilver probably lives up to it. He's snobby. He's rude. He doesn't tip. He dresses in tight clothing. He sometimes wears jewelry (gasp). He kisses people on the cheeks (double gasp). He acts like a wealthy socialite, but doesn't necessarily have the capital to back up his lifestyle. The most important element of his eurotrashiness, however, is the amount of condescension he doles out on people. Put everything together and you've got a man who almost instantly rubs people the wrong way.
Because Quicksilver's powers seem to be entirely physical in nature, the amount of energy that he has for running depends on how many calories he can consume. Simply put, if he doesn't eat a bunch, he can't do much more than jog.
Imagine if your resting heart reate was over 200 beats per minute. Now imagine if you could go to the grocery store and back before most people even finished forming the decision to get groceries. Now imagine waiting on something. Quicksilver has virtually no patience to speak of. His quick decision-making abilities are often an asset, but they're just as frequently a curse. He is possibly the worst candidate for any type of leadership role ever, preferring to do things himself rather than waste all the time developing a complicated plan that utilizes all of his team's assets appropriately. Fortunately, his methodology has managed to produce results, or he would have been discarded by the Brotherhood long ago.
Magneto recruited Pietro into the Brotherhood. It's possible that he did this because of some sort of paternal instinct, but it seems far more likely that he doesn't have any of his childrens' best interests at heart.
Quicksilver hates humans. He thinks they're basically monkeys.
As anyone who has ever said anything remotely rude to Wanda knows, Pietro's fuse is nonexistant. He literally goes from calm to seething with rage in less than the blink of an eye. Insulting Wanda is probably the surest way to infuriate Pietro, but it definitely isn't the only way. Waiting for his team to catch up with him makes him angry. Not achieving measurable results right away makes him angry. Anti-Mutant rhetoric makes him angry. He's basically a Temper Tantrum That's Always Just About To Happen. When he gets angry, he almost always does something destructive and/or stupid. Fortunately, he is not beyond reason, which is why having a much calmer, more rational sister is such a huge blessing.
If someone only spent the time adding up every second that Pietro has spent either actively protecting Wanda or worrying about her, they would come up with exactly 26 years. Pietro wastes so much time trying to take care of his sister that it's almost sad. One day she will almost certainly be the death of him. But aside from the stress that looking out for a walking bad luck magnet entails, it also keeps Quicksilver from being independent. Without his role as the Protective Brother, there would be so little left of his personality that he'd be virtually unrecognizable.
The Brotherhood is considered a terrorist organization by virtually every law enforcement organization in the world. Pietro is in it. Pietro doesn't ever wear a mask. Pietro is therefore considered a terrorist by virtually every law enforcement organization in the world. His real name still isn't known, due to his anonymous Romani upbringing, but Quicksilver is rapidly gaining a reputation as a dangerous criminal.