Name: Ireul, Angel of Fear
Classification: The Eleventh Angel
Appears in Episode: 13
Ireul is the Eleventh Angel, and is the smallest of all the Angels (in direct contrast to its predecessor, Sahaqiel, the Tenth Angel, which is the largest).
Although considered one Angel, Ireul is actually a colony of microscopic nano-Angels that have the capacity for rapid evolution. It is often compared to bacteria, though its behavior more closely resembles a virus. First detected in protein wall 87 and misidentified as a form of corrosion, Ireul quickly spreads into a test chamber called the Pribnow Box, which houses incomplete mockups of the Evangelion Units used for testing and analysis.
After Ireul infests these mockups, it uses them as hosts to evolve into a form of bio-circuitry. Lacking a mobile physical form to cause any direct damage, it instead hacks into the MAGI Super-Computer System (NERV's brain) in its attempts to self-destruct the facility. It successfully overtakes and reprograms both Melchior-1 and Balthasar-2.
Before it can hack into Casper-3 however, the last of the three MAGI cores, Ritsuko Akagi slows down the processing power of the entire system. While the system is still unable to stop Ireul's attack, the Angel is forced to process data at the same slow speed as the system, buying NERV two hours to come up with a plan.
Taking into account the Angel's greatest ability is its rapid evolution (which allows it to constantly change and adapt to overcome its weaknesses), Ritsuko Akagi proposes to use that very ability against it. Since the end result of evolution is self-destruction, she implants a program directly into Casper-3 to accelerate the Angel's evolution and lead it to its evolutionary dead-end (essentially killing it).
With only one second to spare before NERV Headquarters self-destructs, Ritsuko's program is initialized. In an instant Ireul is destroyed, making it the only Angelic threat to be overcome without the use of an Evangelion Unit. Ireul's infiltration of NERV Headquarters is so damaging to Gendo Ikari's plans that he is forced to deny the entire incident to the Human Instrumentality Committee, instead explaining it to be a malfunction in the alarm system.