He was born to a Spanish father and a British mother, having run into each other at a sporting event. Diego Santiago was an auror for the Spanish government at the time, however he was planning to move to London and retiring from his career. However, this was cut short when Paris was five months pregnant with Diego being killed in a hit and run. A mundane sort of death that Diego tended to laugh at. Heartbroken, Paris did the only thing most people would do. Return home and to family. She gave birth to Agustín in London. Growing up, Agustín was raised more by his grandparents than his mother, who tended to hide away in her room, and as a child, tended to believe her more of a stranger than anything. A friendly neighbour even. Despite this neglect, Agustín grew up loved and was treated normally, even when his grandparents found Agustín researching Aurors. Seeing as the grandparents disliked Diego, they were biased when it came to the career he was well known for, they tried to push Sebastiaán down other career paths as he grew up and away from his idealisation of his dead father.
Growing up, Agustín craved a brother, a sister, a father and when he was old enough to understand, a mother. Yes, he loved his grandparents, loved his aunt and uncle and their child, but he wanted a typical family. Like his cousin had. He wanted the cookie cutter life. A normal one. Of course, he didn't tell anyone this. Sometimes, when things got tough, he'd sit outside on the veranda, dreaming his dad would drive up and take him away. At school, he was one of the popular kids. He was good at sport, good at school, and he was the golden child of every classroom he entered. He had his first instance of magic at ten years of age, where he accidentally pushed his grandmother's favourite vase off a counter and stopped it from smashing through his magic. His grandparents were overjoyed and they eagerly waited his Hogwarts letter.
At Hogwarts, nothing changed that much. He was still a golden child and got along with everybody, eagerly joining quidditch the first chance he got. Nothing major happened during the first three years except Agustín found himself enjoying Hogwarts more than his home, finding it empty and hollow. When Agustín was 16, he heard the news that his aunt and uncle had been killed, he wasn't the closest with them but he did have a relationship with them. It was because of this event that his cousin, Karsci, was going to live with them, which Agustín was fine with. With Karsci, he might find something he'd been craving for since he was born. A brother. However, when Karsci arrived, he was distant and hardly spoke a word with Agustín having to work for a week to get a sentence out of him. However, Agustín was stubborn and determined and wanted to at least become friends with him. Agustín understood, of course, that it must be rough for Karsci and probably needed space, but surely it was wrong for him to leave his cousin on his own? At least offer a shoulder for him to cry on.
However, at Hogwarts, it turned around. Karsci and Agustín bonded somehow and were basically inseparable for the next year, becoming the brothers Agustín always wanted. Except...once again, something triggered Karsci to revert to his original state in Agustín's seventh year and he was shoved out again. The other boy was cold and acted as if he wanted nothing to do with him. Getting fed up, the course of action he chose was direct confrontation but this just caused Karsci to blow up at him, accusing him that Agustín had no idea what Karsci was going through and that he wanted to be left alone. Agustín, however, decided maybe it was for the best to leave his cousin alone and so he did. Letting them grow distant and broke their connection that they had begun to build over the previous year.
Agustín graduated a few months after that, getting work as a bartender for the next decade. All the while, he was working on his magic, improving it as much as he could before he attempted to become an auror. Part of him was unsure if he was able to do it, able to meet the requirements of it and wanted to spend some time mastering complex spells that might help improve himself. All the meantime, he distanced himself from everyone in his family. He tried, initially, with Karsci but...it never worked, despite either of their efforts. Sometimes Agustín wanted to and sometimes Karsci wanted to but they never seemed to match up, so everytime it just failed and they eventually both gave up. With his grandparents though, they became bitter that Agustín sought a career in being an auror and eventually Agustín decided it was better for all of their sakes to just break free of them.