Katsuo lived a relatively peaceful life with his parents in Higashi. He seldom talked with his classmates in his elementary school because of his timid nature and only made 3-5 friends. These friends were the only ones Katsuo had until his parents passed away from an unknown disease and he was forced to leave his home in Higashi and his friends when he was 12 to live with his uncle: Hideaki Viperia in the city: Hitoyoshi, in the Kumamoto Province at his parent's request before they passed away.
Katsuo quickly fell into a deep depression and would not talk to nearly anyone at all; he packed his belongings with help from his friend's families and after he gave his quiet farewell to his friends, he then left to his uncle's vehicle to go to his new home in Hitoyoshi. Once Katsuo unpacked his belongings, he quickly learned that Hideaki was a businessman and that his work would keep him away from home nearly all the time. This further drove Katsuo into isolation, because he was in a brand new environment with no friends and no nearby family. Katsuo enrolled in a nearby school to continue his education but made no effort to make friends because of his personality stacked up with the fact his parents died not too long ago.
Ever since Hideaki let Katsuo live in his house, he's been concerned that Katsuo still has trouble getting over his parents' deaths and that he hasn't been trying to make friends. Hideaki did hear of a brand new MMORPG that will be entering beta testing relatively soon, so he used some of his business contacts to acquire a Nervegear, and a spot for Katsuo in beta testing. Once Hideaki brought this piece of news to Katsuo, he was curious about what 'Nervegear' and 'MMORPG' was, so after a little explaining, Katsuo seemed interested and started up the Nervegear to enter the world of SAO for beta testing.
For the first time in around 6 months, Katsuo actually felt alive to a point; the community of the game was small, but a good number of the players were very friendly towards him and his old scars began to heal. Once the beta test ended and the official release was announced, Katsuo asked Hideaki if he would help him get the game once it was released. Hideaki was glad that SAO helped him break out of his shell so he agreed to it. Hideaki paid for the pre-ordered copy of SAO. Once it arrived, Katsuo quickly went to reconnect to the world of AIncrad so he could be reunited with his brave character: Masaru.
When Masaru reconnected to Aincrad, he quickly left to some nearby plains and laid down to relax, remembering the good times from the beta test. Masaru overheard a pair of players: one with longish black hair, and one with red hair saying something about the log out command being gone from the menu. This piece of news interested Masaru immensely so he decided to check if it was true or not. Upon inspecting the menu, he shockingly found the logout being absent, and was about to freak out when he was suddenly enveloped by a bright light. He found himself in the main plaza of the City of Beginnings along with what appeared to be all 10000 players of the game. The creator of Sword Art Online and the NerveGear: Akihiko Kayaba announced that SAO has now become real 'death in this world will be death in the real world'. What surprises Katsuo is that he is not fazed by this in the slightest, even when he sees everyone around him losing their minds and going crazy. When he inspects the mirror and his appearance is forced to his real life appearance, it created more drive for him in a way.
After the announcement was made, Masaru quickly slipped away from the crowd, bought a new weapon and some armor with the Col he had from hunting monsters and left to pursue his new goal "If what Kayaba said is true, we can now die permanently...I must protect as many people as I can. I don't want these people's families to suffer what I went through"
Katsuo has a very idealistic and kind personality, in the beginning he wanted to clear the game and make sure all 10000 players escaped Aincrad unharmed, but he was never able to admit that it was impossible for all 10000 players to escape, even after the first month passed and 2000 players perished. It took a wake-up call from a future enemy for him to admit that his goal is honorable but ultimately impossible, he should use his gifts and strength to clear the game to minimize the losses this game does to the players.
Katsuo's character song is Innocent Sorrow. He likes the song because it reflects his life within Aincrad.