Sachi

Sachi (サチ) was one of the 10,000 players who were trapped in «Sword Art Online» and a member of the Black Cats of the Full Moon guild. She is a fairly timid girl who harbored fear of the game and dying, a secret which she later shared with Kirito. She was Kirito's first love prior to Asuna.

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.

Red-nosed Reindeer
Sachi was introduced as a member of the Black Cats of the Full Moon guild, and welcomed Kirito as he became a new member of the guild.

She was mentored by Kirito to switch to sword and shield from her original profession of a 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 other's 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 on June 22, 2023 (at exactly 5:45:13 PM)  along with most of her guild when a trap, which locked down the area, made multiple monsters spawn around the trap and prevented the use of teleportation crystals, was triggered. After her death, Kirito was left in despair and self-loathing, even more so when Keita committed suicide shortly after the death of her and the rest of the guild.

Months later, Kirito received rumors of an item, which could resurrect dead players, that will be dropped by a Christmas event (which was a month away) boss. Even though the odds of that being low, Kirito decided that even less than a 1% chance of it being true is worth the risk and started training relentlessly for the upcoming event, taking many unnecessary risks and sleepless nights leveling in preparation for the Boss Fight for even the smallest chance of reviving 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 that 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. Just as the message is about to come to an end, Kirito starts to weep.

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. This was also pointed out in abec's blog.
 * 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 this item due to the "alarm" the timer gives out.
 * In the Light Novel, Sachi and Kirito had a shared inventory space, which either one could access at anytime, mainly used to put common items such as potions and crystals for their convenience. The creation of this slot was requested by Sachi under the guise of being able to share potions (even though the guild inventory could be used for this purpose) between them. The real reason why she wanted this slot was to be able to leave a comforting goodbye message to Kirito, in case she died before Christmas.
 * Her death was one of the driving factors for Kirito's refusal to join another guild, and becoming a  solo player, until over a year later, when he was comforted by Asuna and joined the  Knights of the Blood.
 * 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).
 * In a 2005 interview with Kawahara Reki as Kunori Fumi during the third popularity contest, there was a Q&A session with the characters themselves. Sachi was revealed to be Kirito's first love prior to Asuna.
 * In Sword Art Online: Infinity Moment, Kirito's inventory contains items from Sachi (and Asuna) separate from other items. The shared inventory contains a record crystal that can only be activated in Kirito's room.

Sachi Сати