Ichiru Kiryu

Ichiru Kiryu (錐生 壱縷, Kiryū Ichiru) is Zero's younger twin brother. When they were younger, they were both taught by Toga Yagari to become vampire hunters. Ichiru, however did not seem to have the same talent as Zero's natural ability, and was often ill. Zero and Ichiru were very close as children, although later Ichiru claims to have hated Zero after overhearing that his parents preferred Zero over himself (Only because he didn't have the same talent as him).

Shizuka Hio let Ichiru drink her blood to strengthen him and cure his chronic ill health, although she did not turn him into a vampire and he remains a human. He now attends Cross Academy as a Day Class student by the orders of Shiki's father, Rido Kuran (and by Zero's permission). It appears Rido has promised Ichiru that he will kill those who killed Shizuka, which is probably the main reason why Ichiru is working for Rido Kuran.

When Zero was locked up by the Vampire Hunters Association, Ichiru visits him and appears to have shot him due to hatred. However, it is revealed that Ichiru was terribly wounded because he tried to kill Rido, the person he was truly after him because of his role in Shizuka's death. After he shot his brother, he convinces Zero to drink his blood so they could finally become one as they were originally supposed to be. Ichiru stated that he was happy that he wasn't dead in Zero's heart. When they hugged, he says that he feels comfortable to be in his arms, recalling a time when the brothers slept in the same bed often and were loved equally by their mother. After Ichiru's death, Zero appeared to grow vines, most likely from all of Shizuka Hio's blood that Ichiru has ingested to prevent his illness. Zero buries him alongside their parents grave.

Ichiru appears to have romantic feelings for Shizuka Hio, even though she was the one that had massacred his family and turned Zero into a vampire. In a bonus chapter, while standing over Rido's coffin, he said, "Shizuka-sama was beautiful even when she fell"... He'd always begged her to turn him into a vampire but she always refused. Shizuka did not feel the same way, perhaps only because she was too heartbroken over her dead lover, but it can be seen that she deeply cared for him.

Death
Ichiru Kiryu died by request. Before his death, Ichiru begged Zero to bite him and drink his blood. Zero hesitated, and drank his life's blood, leaving no drop of blood left.

Trivia

 * Ichi is the old-fashioned way of writing "one," and ru means "thread."