Unit One is the personal machine of Shinji Ikari. It is a powerful machine shown time and again to have a greater personality, or at least a more active one, than the other Eva units. There is a constant simmering rage deep within the unit which the pilot feels keenly.
Unit One is named -Principio Eternus- by Shinji Ikari, a move designed to impress Rei Ayanami but which nearly destroys its docking bay as Unit One shows its approval of the name. It is the only Eva to bear a name, one of three Epics to do so.
Shinji uses the Eva to do a lot of physics-defying stunts which he eventually becomes famous for and causes Ritsuko Akagi no end of headaches and ranting. This includes homing lasers, riding an angel like a rodeo, and singlehandedly saving every major city on the planet from being nuked at the same time.
"Cast in the shape of man! Made to fight humanities' enemies! You shall carry our will unto eternity! I NAME YOU!" He leaned over the railing. "EVANGELION! I NAME YOU; TITANICUS!