Beast Mode

Eva-02 after completing its reconfiguration to "beast".

Beast Mode (ビーストモード) is a special built-in combat function that gives an Eva pilot access to more of an Eva's innate power, resembling a "controlled berserk".

In Evangelion 2.0, the mode is showcased when Mari pilots Eva-02 against the Tenth Angel. After traditional combat methods prove ineffective, she initiates the process by saying, "Invert mode. Secret code: The Beast!"

This jettisons the shoulder pylons, painfully releases the Eva's limiters, and triggers a physical transformation wherein the Eva "abandons its humanity". The Eva's body proportions are generally elongated, and the head becomes saurian in appearance, with a large maw and uniformly shaped teeth. By the end of the transformation, many of the Eva's outfittings are missing or otherwise altered. The reconfigured Eva is still controlled by the pilot and relies on internal battery power, but exhibits vastly heightened mobility and A.T. Field-breaching power.

Both the Eva and its pilot behave in a very animalistic manner. Activating Beast Mode seems to be very taxing on the pilot, and entails a negative plug depth that puts her at great risk for losing human form herself. The only unambiguous physical change that Mari experiences as a result of the venture are glowing green irises.

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In Evangelion 3.0, Asuka's revamped Unit-02 has a beast mode of its own triggered by giving the verbal command "Code 777 (read as "Triple Seven")". Upon this command, her Evangelion takes a more extreme form of Mari's transformation, going so far as to transform into a giant cat-like creature. Likewise, Asuka gains feline features herself, such as fangs and slit pupils. It exhibits the same extra strength and mobility that Unit-02 does in Evangelion 2.0.

Beast Mode
Eva-02 tearing through 10th Angel's A.T. Field with teeth.	Pilot Mari under influence of Beast Mode.

Name

The term "Beast Mode" is not used within the film itself, but rapidly entered the fan parlance. (First appeared as a a fan coinage?) It appears on the packaging of a set of Eva figurines released by HDM Souzetsu, and possibly other merchandise, which gives "Beast Mode" sufficient authenticity.

The "official" term for the mode, as used by Ritsuko in-film, is Beast Form 2nd Phase (Type 1) (獣化第２形態（第１種）, Juuka Dai 2 Keitai (Dai 1 Shu)). This raises the questions of what "1st Phase" would refer to, or what the other implied "types" are.