Peter is a twelve year old boy. That's the best way to describe him: he's precocious and inquisitive, sneaking around and spying on his older brothers and father to figure out the ways of the world. He's fiercely independent (or wants to be, anyway); fights with his dad always include the phrase “you're not the boss of me!” When he has a problem, however, he has no problem going to any of his family and asking for their help. He will never admit it, but he looks up to his brother Arthur to the point of mimicry. He also wants to run away to an island somewhere so he's always surrounded by the ocean air; at the same time, he wants to become the Minister of Magic (after, of course, having been head of the Aurors for years.)
Roy, their cousin in Essex, rescued Peter from a neglectful home, but didn't have the resources to properly care for such a rambunctious toddler. Thus, Arthur took him in and it was all downhill from there.
Growing up as the baby under Alfred's large, loud shadow has caused Peter to be rebellious, wanting to come out on top and prove to the world (but mostly to his family) that he is the best. He can't help but clash with Arthur due to his wide independence streak, but he is hyper-aware of the fact that he was adopted; this causes him to feel sometimes like a second-class family member, and that contributes to his high ambition and drive to succeed.
His favorite class is Defense Against the Dark Arts, and he can't wait to get into Pre-Auror Training. He doesn't mind being on a broomstick, but it's not his favorite thing to do; he feels most at home in the water, so he's liable to jump in the lake. He also loves Transfiguration and wants to become an Animagus; it's not something either of his brothers can do. Plus how cool would it be to transform?
He tries, he really does, but he's rubbish in both Herbology and Care of Magical Creatures, with the clear exception of the water-based flora and fauna. He has an instinctive feel for how to care for them (and is already familiar with a few for his time on the shore), and wants to learn to speak Mermish.
From the beginning, Peter always looked up to his brother Alfred, and tried following him on childhood adventures. If Alfred spurned him, Peter came up with his own feats: when he was ten, he set his right shoulder on fire trying to do a spell he had seen Alfred do.
He's not as close to his brother Matthew, but he does love him even if he tries to get Matthew to do his own bidding. When he's sad, or hurt, or simply wants a hug, he goes to Matthew; Peter has not yet learned that talking through his personal problems can help, but when that moment does arrive, he's most likely going to go to the only person in his family he finds thoughtful or caring: Matthew. He appreciates his brother, though he'll never show it.
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