Unit Zero is the machine piloted by Rei Ayanami. It has no mouth and can make no sound. For a long time, the unit had a head configured into a complicated sensor and testing array, rendering it nearly useless for combat. This forced it into a fire support role until after Asuka showed up in Unit 02.
Unit Zero, like the other Evas, eventually has Titanus Modules created for it by NNHIS.
Unit Zero is never named, one of the few Titans in the series to lack a name.