3: Design the Game Mechanics - Advantages
Advantages are things that give the character a little bit of an edge and often a lot of distinction. In theory, the list of advantages is limitless. In practice, most of them have similar mechanical traits. This rules packet lays out a list of advantages so that you can either pick one (or more) if what you want is already here or give you a framework for designing your own.
At creation a character starts with at least one advantage, and can get up to two more by taking more disadvantages. Characters can gain or lose advantages over time - some of them anyway - most often in the form of advantageous equipment or allies. Many of the advantages listed here are only available at character creation, as you can't be mis-sorted after the fact.
Athletic Aptitude: the character can divorce her mind from her situation to favor physical skill. She can always add her Agility to the skill in a physical test rather than a Quality. This will not help against magic, so a spell that makes the character flee in fright will still set her running but she will do so with her usual grace. When fighting a scary monster instead of the normal Bravery + Brawl roll she could use Agility + Brawl - likely surprising the Boggart no end!
Beautiful Feature: the character, while naturally attractive, has a single feature - hair, eyes, poise, etc. - that sets her apart as beautiful. The character gains a +3 bonus on all social tests where attractiveness plays a role, can brag endlessly about her beautiful violet eyes or naturally curly hair, and will be cut slack by adults who claim to not be effected by such things. This can be taken twice, meaning that all of the character's features are beautiful and doubling the bonus, but she must also have the Half-Breed disadvantage. Fleur Delacor has this advantage twice as a quarter-vela.
Common Mastery: The character is exceptional at a single Common skill, giving it a +8 bonus. This is sometimes inherited and sometimes a personal quirk. You may select the skill or ask the GM to roll randomly to determine it.
Familiar: Pets are allowed in Hogwarts: Cats, Owls, Rats & Toads are common. These are normal wizarding animals, but with Familiar the character has a superior animal. Perhaps the animal can speak, or maintain empathic or even sensory contact with their owner. The animal may simply be more versatile - such as a clever monkey - or potent, such as a large dog or snake. This can be taken twice, meaning that the character has (in violation of several rules) brought in a magical creature like a shoulder dragon or her own house elf. Think very carefully before giving the GM an opportunity to play a house elf where your character will be held accountable for its actions. The GM has to approve any familiar. Lord Voldemort has this advantage in Nagini.
Famous: The character is, for some reason, famous in the wizarding world. As an advantage this means that she has +4 with Ambition checks where her renown would play a role (Fame is also a Flaw, see below). This is very rare for a Hogwarts student. Albus Dumbledore is Famous, as is Harry Potter. You must give a reason for the character's fame, but for first through fifth year students this is best explained by her being part of a family with long standing fame.
Large Family: the character has a large family that she can call upon for assistance, at least two of whom are in the school. Family members provide indirect help rather than directly resolve plots; based on personalities they may be an occasional hindrance. You do not need to take this advantage for player character siblings, and if a pair of PC siblings wants a large family only one of them has to take it. The character who takes it is a favored sibling while the other one is a black sheep, but the black sheep can call on the family in a stretch. If both take Large Family than the family size is doubled! Ron Weasley has this advantage.
Large: The character gains a +2 Size (first year max is 5). In addition, roll to see if her size increases twice a year. This can be taken twice, you must also take the drawback Half Breed - Giant, like Hagrid.
Magical Aptitude: the character excels at a single Magical skill, giving it a +6 bonus. This sort of skill is often passed down through families - Harry inheriting his father's skill with a broomstick - but it can be unique to her - Neville Longbottom's skill with herbology. You may select the skill or ask the GM to roll randomly and discover the predisposition when the character does.
Magical Widget: The character has a magical device that can be reliably used for a significant effect. This has a lot of options and needs to be cleared by the GM; you need to provide an explanation for how the character got it. A broomstick does not qualify - not even a Nimbus 2000 - as they are too common. Harry Potter's invisibility cloak is the most noted Magical Widget in the books, but Hermione Granger's Time Turner and Draco Malfoy's Hand of Glory also qualify.
Mis-Sorted: the Sorting Hat placed the character in contrary to her best house quality. This means two things: first, the character's house quality may be her second best stat; second, her house quality goes up 1 point. For example, Hermione Granger rolled Ambition 2, Bravery 2, Fortitude 2, Intellect 5, Agility 2, Size 2. With Mis-Sorted she is placed in Gryffindor rather than Ravenclaw, and her Bravery increases to 3.
Muggle Expertise: The character has a surprising facility with a single Muggle skill giving it a +12 bonus. This is obviously most common among Muggle-born, and will likely be of limited utility in play. You may select this skill or ask the GM to roll randomly and be surprised when the expertise appears (if it ever does!).
Perfect Wand: The character's wand is a perfect match to her magical abilities. Such matches are rare, and the wands are often expensive (though money is no guarantee). Most first years have hand-me-down or inexpensive wands, but those with a Perfect Wand can add +1 to any magic test that requires wand use. Harry Potter has this advantage, as does Lord Voldemort.
Teacher's Pet: For some reason one professor at Hogwarts gives the character preferential treatment. The reason might be established by an event early in the character's time at the school, it might be because the professor is friends with the character's family or you might have no idea! Whatever the reason, it gives the character a powerful ally to call on. The genre rules mean that - as with a large family - the teacher ally won't solve the character's problems, but it will smooth out some of her rough times. You may pick the teacher or let the GM do so. Harry Potter has this in Dumbledore, but for this game the headmaster is off limits.
Wealthy: The character's family has access to above average wealth in the wizarding world. This will provide a variety of social advantages and superior equipment, including a +3 on Ambition rolls with people who are ether aware off the character's wealth or are swayed by her obvious displays of funds. Draco Malfoy has this advantage. You could argue that Harry Potter should have it, but he doesn't because he hides his wealth.
Well-rounded: raise your lowest statistic (just one of them if two statistics are tied) by 1 point, indicating that the character has is well balanced in her statistics.
Player Designed: Hopefully this list will give you an idea of the range and mechanics of Advantages. I will note that advantages that boost stats will always boost lower Stats rather than higher ones, as a mechanic to avoid too many high statistics.