Marcus was raised by his parents, Pater and Desdemona (nee Crouch) Flitwick, and is young enough to have grown up as all the furore surrounding Voldemort's return was reaching its peak. As a result, his parents were always cautious about their relationship with others of the wizarding kind; his father took a muggle job as an accountant, and despite occasional contact with his uncle Filius, Marcus grew without much knowledge of the wizarding world, lest it place him in danger.
His magic manifested when he was six, and he was so desperate to taste the drinks his father and uncle were sharing on one of their occasional meetings that his mouth instantly filled with a rather unpleasant blend of firewhisky and butterbeer, about half of which he swallowed- briefly- and the other half of which was quite enthusiastically sprayed out of his nostrils. His parents, and his unusually gifted uncle, immediately took on the task of not only training him in the use of his magic, but also- and far more importantly- the ways in which it could be hidden. As a result of this early training, Marcus developed a faintly solitary and secretive nature, often writing little plays which he refused to let others read.
Six months after Voldemort's final defeat at the hands of Harry Potter, Marcus' parents finally deemed it safe to let him mix more freely with other wizards, but he did not make any fast friends until he finally went to Hogwarts (two years after the events of Deathly Hallows.) One of the things that contributed to his solitude was the lack of knowledge he had about the events of the past few years- his parents had gone out of their way to ensure he was not informed about them, and so the tale of Harry Potter and Lord Voldemort seemed a little overblown when he heard the stories third and fourth-hand. This faintly skeptical attitude towards the events of the past few years tended to earn him slight dislike from those young wizards and witches he was introduced to.
When it was finally time for him to go to Hogwarts, Marcus spent the entire time on the train buried in 'Hogwarts: A History' and the other course books, barely noticing the stunning countryside or the other children; as a result, since he had made no friends at all, he sat with Hagrid as they took the boats across the water. He was sorted into Slytherin after a full minute of deliberation from the Sorting Hat, and seemed as happy with the selection as he would have been with any other.
His time at Hogwarts was not easy, however. Despite excellent marks in Arithmancy, History of Magic, Potions and Charms, his other subjects tended to suffer due to the intensity with which he devoted himself to his favourite areas, and also to his habit of writing short pieces of fiction in the middle of classes. In the second year, he discovered the room of requirement when trying to find somewhere pleasantly quiet to read a book, and to this day thinks the room is a pleasantly warm library, with a roaring fire and deep, comfortable armchairs. He also suffered from Slytherin's terrible reputation, since he was the shortest and smalllest boy in his year; a tendency that would become more and more pronounced as time passed, since he reached the limit of his height, 5'3", in the fourth year.
Continual bullying, both from those who hated the house and those within it who were frustrated, left Marcus slightly bitter as he continued his education. It was only when he was invited to one of Slughorn's parties, in the fifth year, that he began making friends, and was soon firmly entrenched as a "boy going places!" by Slughorn. In his final year, due to a lack of players and some cajoling from Slughorn, he joined the Quidditch team as seeker; they finished third, and Marcus personally scored two snitch captures. He's always slightly amused he ever played.
After he'd achieved his NEWTs (Os in Arithmancy and Charms, Es in History of Magic and Potions, and an A in Astronomy) he decided that he would best like to teach, and so utilising his contacts in the Slug Club got himself a job at Gringotts, where he worked in accounting for several years, in order to gain the experience necessary. During this time, he continued to keep close links with the Muggle world, and is competent with computers and the use of other Muggle technology. He's also fascinated by the martial arts that the Muggles have invented, and practices Tai Chi to this day as an efficient form of exercise.
3 years ago, Marcus applied for and received the job of Professor of Arithmancy at Hogwarts, and has a reputation as a good teacher but a bit of a hardass. He doesn't like to be interrupted, but he does try to make Arithmancy at least interesting if not exactly fun. After Slughorn was promoted to headmaster, he was appointed Head of Slytherin as an interim measure, and then permanently. He enjoys the responsibility of the position, and particularly works with all his students to promote inter-house unity (which he feels is important simply to defend his house, if not actually worthwhile as a goal in it's own right). An important note is that under his command, Slytherin has had no less than three muggle-born children accepted into it, which he feels will go a long way towards salvaging the house's reputation. He lives in Hogwarts full-time now, and rather enjoys exploring it when the students are away during the holidays.
Now, onto a quick note about Arithmancy, because no-one knows what it is. Marcus’ Arithmancy course is split into three important areas:
1. Using magic like a pocket calculator, to perform arithmetic, algebra and calculus, and a complete understanding of these mathematical areas;
2. Studying the underlying mechanisms of magic, and learning the equations that show how a spell is powered and cast;
3. NEWT level only, and for those who also show an interest in Muggle Studies: Combining Arithmancy with computers, and magical use of the interweb.
Marcus’ classroom is bright and airy, and the desks are spaced reasonably far apart. He attempts to keep a maximum of every other lesson being wands away, but with so much basic maths to cover, early on it can be difficult. He’s also invented a couple of minor jinxes he used to use as a student, and tends to gleefully teach them early on in an attempt to stoke up an interest in the subject. These are the Fibbonaci jinx, and the Auferonumerus jinx. The first forces a victim to think through the Fibonacci sequence, and progresses from a slight irritant to having to sit down and think very quickly- it can be easily shaken off by learning to think two steps ahead. The Auferonumerus jinx simply removes all knowledge of numbers from the victims mind for a short time, which doesn’t sound so bad, but forgetting how many legs you’ve got tends to trip people up. Both are banned during lessons, obviously.
Marcus is, these days, mostly polite and pleasant, and deeply enthusiastic about his subject. He's evasive but not rude when asked about his home life, or his upbringing. He can be quite vicious when he is conversing with someone he dislikes; he finds it difficult to avoid attempting to goad them slightly, a bad habit he picked up when being bullied (when feeling threatened, part of him assumes there will be a fight whatever he says, and so he tries to get his digs in with words). He likes to feel he's earned things, and tends to be slightly dismissive of those who have not; for example, he was pleasantly surprised when he found out that Harry had taken the extra year to complete his NEWTs, rather than just taking the job he was offered by the ministry.
Character's Appearance
Marcus is a tiny little man. He's only 5'3", and is very slight; as a result, he has taken several measures to ensure he is not confused with the students. He keeps his straight brown hair short, and wears a vandyke beard (goatee with moustache, Jafar-style). He also wears grass-green robes with clear silver trim; honestly he considers them a little garish, but it's a small price to pay to distinguish himself from the children. He also has a habit of docking points in an incredibly severe fashion from the house of any prefect who makes the mistake of giving him an order.
He wears a silver ring with the family crest on his right hand, and carries around a beaten-up leather satchel in which he keeps his notes, and some unfinished writing. His voice is deeper than one would expect, and gets softer and quieter the angrier he is. His eyes are a clean hazel, and he would probably be considered quite attractive were he not so small.