Yumizuka Satsuki, also known as "Sacchin" is an important side character in Tsukihime and Melty Blood (from Re-ACT onwards).
Satsuki is a classmate of Shiki Tohno, as she has an unrequited crush on the boy after he saved the lives of her and her sports club from a locked freezing room during a winter day. One day, Satsuki works up the courage to walk home with Shiki and have him promise to protect her should she be in any kind of trouble. While out at night alone to investigate rumors of Shiki being around town, she is bitten by Roa under the guise of Shiki TOHNO and becomes a Dead Apostle.
Satsuki stands at roughly 63 inches tall (161 cm) with her three sizes being 79 cm for bust, 59 cm for waist and 82 cm for hips. She has brown hair and eyes (red eyes while a Dead Apostle), and is regularly seen throughout Tsukihime appearing in her school uniform with her hair tied in twin pigtails.
Satsuki's personality varies between adaptations. She is shy and has somewhat of an inferiority complex in the visual novel, while being more outgoing in the anime. Satsuki is popular among her classmates, though she doesn't pay much mind to this status.
Satsuki's role tends to vary in differing visual novel routes and media adaptations for Tsukihime.
In Arcueid and Ciel's routes in the visual novel, she is reported to have disappeared and it is strongly implied that she never became a Dead Apostle in her encounter with Roa, thus having been killed after having her blood sucked. The anime adaptation of Arcueid's route has her not encounter Roa at all and being among one of Shiki's friends, even confessing her feelings for him near the end of the series. Her friendship with Shiki is retained in the manga adaptation, as her implied death at a hotel (where it is later revealed she never went to) from the slaughter of Nero Chaos led Shiki to develop the drive to confront the Dead Apostle.
Satsuki gets a major role to play in Akiha's route in the visual novel when Shiki discovers she has become a Dead Apostle after discovering she has feasted on the blood of four people. The two meet the following day at a park where Satsuki confesses her feelings to Shiki and had loved him since he rescued her from the frozen room, yet learns he never took notice of her. Coughing up blood, Satsuki bites Shiki's neck in an attempt to make him into a vampire, yet the bite has no effect on him. Begging him to help her, Shiki is forced to mercy kill Satsuki and apologizes to her for his failure to help her as she is dying. The events would haunt Shiki for the remainder of Akiha's arc, as he tries to prevent similar tragic developments from befalling Akiha.
In Hisui and Kohaku's routes, the maid for each route would inform Shiki of Satsuki seeking contact with him. Both routes feature Shiki meeting with Satsuki and not reciprocating her feelings for him, driving Satsuki to bite Shiki's neck to make him into a vampire. However, his Nanaya blood reacts instinctively to this threat by driving Shiki to kill Satsuki in both scenarios.
Satsuki is a very rare occurrence for a Dead Apostle in that not only did she become a vampire within a very short amount of time, but she also gained a Reality Marble, a feat that only the strongest of Dead Apostles can accomplish. Her Reality Marble, Depletion Garden, rapidly drains and dissipates the mana within the area of which the Marble takes effect. This makes it a very effective tool against Magi who need mana to cast attacks and elementals who need mana to maintain their existence. Beings that don't require mana for any purpose are unaffected by Depletion Garden.
Like many vampires, Satsuki also possesses the Mystic Eyes of Enchantment, which allow her to hypnotize anyone within eye contact of her.