The Dead

① After having its blood sucked, the natural process of being reborn as a blood sucking species is skipped and the body is used and manipulated as a Dead Apostle's bunshin (second body - body clone).
② No more than a puppet, so it's not a proper member of the blood sucking species.
③ Obtains energy needed to live by eating humans. However, over half of the energy consumed is transferred to the Dead Apostle it is connected too.

Development Process
① The Dead have had their blood sucked, so the dead go through the same process of becoming a ghoul (a corpse eating demon), and then a living dead (a living body), and then finally become a proper vampire. Vampires have been known to kill their Dead Apostle progenitor, thereby becoming a new Dead Apostle themselves.
② The higher the latent potential of the body, the more likely one is to actually make it through each step of the process and actually become a full vampire. In extremely rare cases, most of the steps required to become a vampire can be skipped. This can be seen in people with extremely high latent potential to become vampires, such as Satsuki Yumizuka, who almost immediately became a full-fledged vampire.
③ A Dead Apostles power level is influenced by the one who sucked their blood.

Colorful MOON Tsukihime: Tsukihime Glossary