Otherwise

Although Sinon is a girl, she is serene and prudent, but at the same time she can get angry very easily, as when she found out that Kirito wasn't a girl. She also proves to be a helpful girl, as she helps anyone regardless of the circumstances.

At the age of 1, Sinon was involved in a car accident in the GGO capital city of Gale that left her father dead and her mother mentally fragile. This led Sinon to develop a sense of responsibility for protecting her mother over the years. At the age of 11, while she and her mother were visiting a post office, a group of terrorists stormed the post office, threatening the staff and other customers inside, including Sinon's mother. This caused the young girl to pounce on the terrorist leader, causing him to drop his weapon. After that, the gun fell into Sinon's hands, and in desperation, she fired three shots at the terrorist leader and then at three of his accomplices, resulting in her being wounded in the process. Before the remaining terrorists could kill her, the police arrived to help the civilians. After realizing what she had done, little Sinon suffered a great trauma that left her with a deep fear of guns.

While in high school, Sinon met Endou and other classmates with whom she felt comfortable for a time, but then they began to abuse her trust, using Sinon's apartment to throw parties without her consent. Upon realizing this, Sinon ended the relationship, which caused her former friends to begin harassing and abusing her, exploiting Sinon's fear of weapons to threaten her and extort money from her.

During her school years, Sinon also met a boy named Kyouji, with whom she got along very well, and who introduced her to the world beyond the walls of the capital city, Gale. Sinon was fascinated by the world outside the city; despite its hostility, she found it beautiful and wanted to explore more. Noticing that her fear of weapons wasn't negatively affecting her, she began to take it seriously, hoping to overcome her trauma.

One day, while Sinon was exploring a dungeon, she encountered a high-level monster, but by then she was already an experienced sniper with military training, both in close combat and long-range combat, specializing in sniping, so she had no trouble defeating it. That's how she obtained her Hecate II, a legendary weapon from an ancient era, which happened to be a sniper rifle.

At the beginning of her military career, she first appears as part of a 5-member squad led by Dyne, where her role was that of a sniper. Sinon becomes involved in a fierce battle with a high-ranking criminal known as Behemoth and his team who terrorized the main trade routes to the capital city, using a mini-gun. After a hard-fought battle, she is one of the few survivors of her team.

Another day, while heading to register for a tournament called Bullet of Bullets or BoB, a survival game where all kinds of people could participate to test their skills against others, she first encounters Kirito, mistaking him for a girl and offering him her help. Later, during the preliminaries, Kirito clarifies that he is a boy, but Sinon can't help but feel deceived and used, so she becomes deeply angry with him and vows to get revenge by killing him herself in the tournament. When Sinon advances to the final phase of BoB, she notices something strange about Kirito and realizes that his concerns outweigh her own. In this way, Sinon ends up allying with Kirito in the final round as they try to figure out how to defeat Death Gun, one of the most wanted and dangerous terrorists in the world who has a conspiracy underway to affect the entire capital city. Hiding in a desert cave with few participants remaining, she tells Kirito about her incident that traumatized her with firearms, using the Type 54 "Black Star" rifle—the weapon of the terrorist leader who, with his group, raided the post office when she was a child. Later, she uses her rifle to cover Kirito from Death Gun, and together they stop Death Gun and his plans, working as a team in their toughest battle yet, emerging victorious and killing Death Gun.

Once Sterben/Death Gun is eliminated, Sinon and Kirito decide to forfeit and voluntarily leave the trial, losing their place. However, in the records and for their heroic act of saving the capital city from Death Gun's conspiracy, the organization that held the tournament registered Sinon and Kirito as winners, both in 1st place. After the tournament and celebration ended, Sinon realizes that Kyouji Shinkawa, her supposed best friend, is actually an accomplice and younger brother of Death Gun and his terrorist organization. Kyouji tries to assassinate Sinon in her apartment, but Kirito arrives to save her, knocking Kyouji out. This leaves Sinon shocked by the betrayal of her supposed best friend, Kyouji, but she understands that her ex-friend now faces a life behind bars, if he's lucky. Feeling grateful to Kirito for saving her again from certain death, she trusts him more, increasing her affection and reliance on him.

Later, Sinon meets with Mayor Seijirou Kikuoka and Kirito to discuss what happened with Death Gun and his accomplices, along with their organization called "Laughing Coffin," leaving the internal task to the intelligence department of the city to which Kirito belongs and to the army defending the city to which Sinon belongs. After this, she is taken to Café Dicey where she befriends Asuna and Lisbeth, two of Kirito's friends, spending a pleasant and friendly evening where everyone got to know each other. The night ends with everyone going their separate ways, but Kirito and the others promised to see each other occasionally, whenever everyone could, saying "see you soon." Sinon was a little sad knowing she wouldn't see her friends for a while, but Kirito and the others promised to see her again and wished her the best on her journey, something that cheered Sinon up, and with a smile on her face she said goodbye to her friends.

Sinon has fine, pale blue hair that falls casually short, but with a strand tied back on either side of her forehead, and indigo eyes. In combat, her attire consists of a desert-colored military jacket, bulletproof armor, and matching combat boots, and she wears a white scarf around her neck. Her measurements are 80-61-81.

• Despite being so young, he has good aim with firearms and, above all, superior accuracy with a sniper rifle, mastering almost all types of sniper rifles almost perfectly.

•Although Sinon prefers ranged combat, she is skilled in hand-to-hand combat, both unarmed and with bladed weapons, something that has gotten her out of more than one ambush or predicament.

•Sinon is very agile and fast, being able to glide and propel itself along cables, which allows it to move faster and move more fluidly in confined spaces.

•If not, possessing a high intellect, he analyzes his enemy before attacking and sometimes during the battle, thinking about the best and most effective way to defeat his adversary.

• Sinon is very resistant to blows or falls; although they may cause him pain, he has a higher pain tolerance than other people, which helps him continue fighting even with injuries to his body, as long as they are not life-threatening wounds.

• It offers greatly improved vision, allowing you to visualize your target up to 3 km away, and with a sniper rifle, you can see it from over 5 km away.

•Otherwise, they possess a great survival instinct, which gives them a certain advantage in dangerous and unknown environments; they know when they can overcome an adversary and have the advantage, and when they should retreat and better plan their strategy.

• Sinon managed to place first alongside Kirito in the third BoB tournament. In the fourth tournament, she came in second after losing to an unknown opponent, something that frustrated her somewhat but also motivated her to improve.

•During her months defending the capital city, she participated in Operation Spearbreaker, an operation where she and a company of her army comrades dismantled a ritual of the Laughing Coffin organization that involved summoning an ancient creature capable of endangering an entire continent. Luckily, and narrowly, she and her company emerged victorious from that battle, preventing the summoning.

Although she had a phobia of weapons since childhood, her friend Kyouji supported her in joining the army, which helped her overcome her fear of weapons.

After the third tournament, Bob Sinon secretly began practicing his skills with laser pistols and swords, weapons used by Kirito in the tournament, but he could never get used to the combination of both weapons, keeping them in his apartment.

Although she isn't very expressive in showing feelings of appreciation or affection towards others, deep down she cares a lot about the people around her. She showed her appreciation for Kirito after their night at the Dicey Cafe, smiling gently and sweetly and hugging Kirito, hoping to see him again before saying goodbye.