Petros had always been a little more on the Surly side, as the youngest he often felt left out and like he doesn't fit in. He can be a bit stand offish and prickly but he is able to at least pretend to be sociable.
Petros' mother was an american muggle who met Atlas while he was on the East Coast for work. Petros and his mother moved to England to be closer to his siblings and his father.
Petros felt left out due to his age and how he toddled after his siblings.
Petros went to Hogwarts up until his fifth year and then his mother died and he attended Tally and lived with Eudoxia and her husband. The rest is up to you. None of the siblings have the same mother and so their experiences growing up could easily differ. If you have questions let me know!