Haruka was orphaned at a very young age with her mother passing away during childbirth and her father dying of an unknown disease a month after she was born, which is presumed to be stress related.
She was raised in an orphanage in Kielce with two dozen other children in situations like hers, with Haruka befriending four of these children during her time in this orphanage. When she turned seven, Lady Luck gave a degree of favor her way and was adopted by Hector & Amelia Uchimaya, who are some of Europe's most talented counter-hackers, who are people that work with security organizations and teams to help prevent cyber hacking of any kind. After a few years pass, with Haruka being hand-trained by her foster parents in the art of counter-hacking to the point where she's nearly as good as her parents, the Uchimayas received an outstanding offer.
The Japanese company: Argus hired the Uchimaya family to help prevent hackers from trying to manipulate the system during the beta and official release for their own benefit. Although, Haruka, Hector, and Amelia can't speak Japanese, they did find a translator that could speak both Polish and Japanese, so the Uchimaya family were able to negotiate the contract for security for the SAO game. During the official release of the game; Haruka, Hector, and Amelia were inside the game overseeing some final security diagonistics of the Cardinal program when they were forcibly teleported to the City of Beginnings.
When Kayaba Akihiko, the creator of SAO, informed everyone of the change to the game's programming, Hector and Amelia were outright pissed off swearing vengeance against him and Argus through their own means and that of the various nations the Uchimayas have helped over the years. Hector and Amelia wanted their foster daughter: Haruka, who went by the name Mikuya in the game, to remain in the City of Beginnings where it was safe while they took the risks to clear the game.
However, Haruka couldn't stand the thought of her living relatively peacefully while everyone she cares for and her family could potentially get murdered by these monsters. She refused to stay in the City and be safe when everyone else was willing to risk their lives, so she chose to leave and fight with her own power, so that if she did fail to save someone, she would have the knowledge she did everything she could to save them.
Although Haruka has never played an MMO before SAO, she was taught the skills of rapid adaptability and strong computer knowledge by her foster parents, so she figuratively stands a fighting chance against this world as long as she works together with the other players. During her time of the first month, Haruka's world was getting shattered into millions of pebbles as two thousand people died in a single month with her failing to save atleast a dozen of those people, with her painfully seeing her foster family get killed before she is saved by a nearby group of players. Knowing that she was powerless to save these people she saw die, she vowed to become stronger so she could do what she couldn't now: 'Protect the people, no matter what.'
When she became active among the clearers, many were shocked that such a young child was willing to do this, where her life would be constantly in danger on the frontlines. The clearers often try to assign her work away from combat heavy zones like exploration and bosses to keep such a young girl from fighting, but she often confronts them and forces them to let her join.
Haruka is relatively withdrawn as a result of her mildly timid personality and the fact she has seen so many people die in front of her. When she's fighting, she's capable of sealing away her emotions entirely so she can fight with no regrets, but when the fight ends and if there are casualties among the players, she needs time to recover mentally from the losses she was not able to prevent. As time goes on, she develops a strong pushy mindset, being able to convince nearly anyone to let her do what she wishes, which she uses to perform reconnaissance and fight bosses, to the clearer's mild reluctance.
Haruka's character song is Connect because of how she and all the other players are imprisoned in a world they may never be able to escape from. She wants to work alongside as many people as possible to clear the game, while she herself is conflicted with her own sadness of being trapped in the game and losing her parents a second time.