Sachi

Sachi (サチ) was one of the 10,000 players who were trapped in «Sword Art Online» and a member of the guild Black Cats of the Full Moon. She was a fairly timid girl who harbored fear of the game and dying, which she secretly shared with Kirito.

Background
Offline, Sachi, along with all the other members of the Black Cats of the Full Moon (excluding Kirito), were in a computer club at their school. She also happened to live fairly close to the leader of the guild, Keita.

Sword Art Online
Sachi was introduced as a member of the Black Cats of the Full Moon guild, and welcomed Kirito as he become member of the guild.

She was mentored by Kirito to switch to sword and shield from her original profession of lancer, but could not handle it due to the closer range to monsters and her fear of battles. They became very close to the point of sleeping together in the same room; their relationship described as "two stray cats licking each others' wounds" as they comforted each other. Their relationship was close enough that they had a shared page in each others' inventory (similar, but not quite at the level of married players who had combined inventories). Kirito promised her that she would not die, though unfortunately, he wasn't able to keep his word.

She was killed along with most of her guild when a trap was triggered that called multiple monsters and prevented the use of crystals. After her death, Kirito was left in despair and self-loathing, even more so when Keita committed suicide shortly after her death.

Months later, Kirito received rumors of an item which could resurrect dead players from a Christmas event a month away, and trained relentlessly for it, taking many unnecessary risks, in his quest to revive Sachi. Having obtained the rare item Divine Stone of Returning Soul after much hardship, to his disbelief and horror, the item had to be used within ten seconds of a player's death, rendering the item useless for his purposes of reviving Sachi, who had died months ago, compounding his grief even further.

A few hours later on Christmas Eve, a time-delayed message left by her opened up in Kirito's inventory. The message told him she already knew of his higher level, along with telling him not to blame himself for her death, which she had predicted would happen due to her nature. She requested Kirito to survive in the world of SAO, and to find the reason why such a world was built. The song "Rudolph the Red-Nosed Reindeer" was also sung by her and her final goodbyes gave Kirito the strength to live. Her death was one of the driving factors for Kirito's refusal to join another guild.

Trivia

 * In the Light Novel Sachi sang Rudolph the Red-Nosed Reindeer with lyrics in it, while in the anime she just hummed along the melody.
 * Fans have pointed out that Kirito and Asuna's adopted AI daughter, Yui, bears a resemblance to Sachi.
 * In the anime, Sachi made a record crystal and sent it to Kirito through a time-triggered mail to be received on Christmas Eve of 2023. Contrary to the Light Novel, where Sachi left the record crystal in a shared inventory tab between her and Kirito with the timer-activation mode active. Kirito noticed about this item due to the "alarm" the timer gives out.
 * In the Light Novel, Sachi and Kirito had a shared inventory space for which both can access anytime, mainly used to put common items such as potions and crystals for their convenience. Sachi would also leave a record crystal containing a message for Kirito on the shared inventory tab.
 * In the Blu-ray/DVD package volume 2, the character song CD will feature Sachi (Hayami Saori) singing "Memory Heart Message" as the 2nd Character Song for this series. This CD is bundled along with the 3rd Character Song, which sung by Silica (Rina Hidaka).
 * Some people said that in Episode 25, in Dicey cafe to be exact, there is a person with resemblance to Sachi joins up in the party. In fact, the girl mentioned is no less than Suguha, Kirito's little sister, and have nothing related to Sachi. For Sachi herself to be there is also impossible, since she already face her death by the time she killed inside the game.