Okay, well, he was born in a poor Muggle family, of which compressed of his older brother, his overbearing Tia mother, and his farmer/cook of a father. You see, this made things very stressful on both boys, and Katsurou even acted on it - via magically means actually. The boy literally slapped his father around with flying fucking fish for fuck's sakes, and magically threw a fit when his father tried making him kill a life prey in front of their customers, and did numerous things throughout his childhood that were indictive that he was indeed a Wizard. Not that his father noticed; he was off making his older brother 'a true man' or whatever. In due course, a letter came to him from the local Japanese Magical school - he still remembers it to this day, getting his murky hands on the golden letter. Before that, he was off helping his brother and father collect rice off of their rice paddies, which is admittedly hard work, besides for you gutting them, which he did ever so often. Hey, a guy had to have lunch, dinner and breakfast right? Well, that's usually what Katsurou's menu consisted of until the school saved him from such a horrific fate. At the school, they served more than just bloody fish and rice - although they were extremely popular dishes at the school, to be perfectly fair.
When he arrived at the school he worked super hard to get to where he was - blood, sweat and tears, all of that. He also failed hard in Karate during his first and second years. But alas, his schooling in Japan isn't a vital part of this story, let it be known, and it was very hard on him.
As the Japanese view family as something fundamentally that they must impress and honour, Katsurou decided he'd rise above the rest of them, farmer boy or not, he was not something for those snobby older boys to stop all over, despite his numerous failings. He'd do it. Or at least try doing it - I guess that's an achievement, the act of trying at least.
He perceived through school; learned English (still working on that though) and how to properly use a sword (Karate and that other shit was made for people whom fought with swords, but I might be wrong about this so don't hold my words as the utter truth of the matter), and other magical stuff. He was particularly adept at Défense Against the Dark Arts during his stay there, due to his strong Patronus and love of learning in general, he was able to best most of other students, and got the highest grade in that course.
Okay, so he asked to go to Hogwarts. Asked for the opportunity to make his family proud, and to explore the world - and since he had strong level E grades in DATDA and was a great student despite the overwhelming competition, he was allowed to go. He wonders how Hogwarts will be, since he's never been there, but he's heard great things about the English Magical School from all the things he's read about them on paper, at least. But paper is much different from real life, as It so happens, and whom knows how his experience in the high halls of Hogwarts shall go?