Yukidoh Sayaka

Yukidoh Sayaka (born 22 February 1990) is Yukidoh Satoru's younger sister and fraternal twin. She appears briefly in a flashback in the prologue, but is never mentioned again in the main story. Her existence is revealed in once the player has collected all other TIPS.

According to the TIPS, she was diagnosed with Dissociative Identity Disorder. She died in 2001, 11 years prior to 2012. The nature of her death isn't described in detail; she's merely described as "a person who is no longer in this world". Her death is the reason why Satoru devised the YUKIDOH Plan, making her central to the mysteries of Remember 11.

Death
Considering Satoru's intense hatred for "That Guy", it's very likely That Guy had a hand in Sayaka's death. This may have been done in one of two ways. In either case, her body eventually becomes Inubushi Keiko. This is supported by their similar appearance and identical blood types.
 * 1) During the first playthrough, Kokoro describes Inubushi Keiko, then a flashback of Sayaka is shown. Many players likely thought the girl in the flashback was a younger version of Inubushi. The player has great influence over the world, and this misconception may have deleted Sayaka's personality.
 * 2) Alternatively, there was meant to be a choice in the prologue. The player's choice would then control Satoru's body can cause him to do something that "kills" Sayaka.

The goal of the YUKIDOH Plan is to get revenge on Self for "killing" Sayaka, and maybe attempt to revive her.