Berserk
Berserk (暴走, bousou) is a state an Evangelion can enter into in which she has gone beyond Nerv's ability to control her, entering into a savage battle-frenzy in which the Eva's strength and ability levels drastically increase. In most cases, a berserk Evangelion can, inexplicably, continue to function even after her internal batteries have been drained, i.e., remain operational beyond the established time limit. Characteristically, during "berserker mode" those Evas with jaws bolted shut will violently force them open and bellow wildly, and show other animalistic tendencies in their movement
During these incidents, the Eva's resident soul appears to bypass restraint mechanisms and wield her own body, although it's arguable how much "conscious control" the soul has over the Eva's actions. (The discrepancy between the behavior of Yui Ikari — prior to her "death" — and her Evangelion incarnation is often cited to this effect.)
"Berserker mode" is very poorly understood by Nerv scientists, and knowledge about it workings limited to such a degree that they essentially openly admit they have no idea what makes it possible, or how it functions. No attempts at artificially initiating berserker mode were attempted by Nerv, quite possibly due to the extreme risk of an uncontrollable, overpowered Eva. Nor, even at the utmost need, can an Eva pilot voluntarily initiate berserker mode: it's simply something that happens when, and only when, the soul contained inside the Eva feels that the need is great enough to warrant it. Not only does Nerv not understand how berserker mode is initiated (or even try to initiate it on their own), Nerv scientists can offer no logical explanation for how an Eva can function after its internal power batteries are drained. Even science division leader Dr. Ritsuko Akagi herself can offer no plausible suggestions as to how berserker mode operates: when Eva 01 moves on its own in Episode 01, Ritsuko simply says that "that just can't happen". "Berserker mode" operates in defiance of the current level of human scientific knowledge, and as Ritsuko says of the Angels in Episode 04, Nerv is truly dealing with beings beyond the grasp of modern science's comprehension.
The fullest extent of berserker mode is when an Eva loses control and continues to function, even after its umbilical power cord is severed and its internal power limit has been exceeded. Sometimes an Eva can go berserk and the umbilical cord is still attached to the Eva (i.e. Eva 01 in the battle against Sachiel), but presumably it is not really necessary at that point. Eva 01 also once moved without power, and also without entering a combat frenzy (to shield Shinji from falling debris, see below), but this is the only instance of an Eva not running amok when it enters "berserker mode".
Eva-00 has two incidents (in Episode 05 and Episode 14), both occuring when a pilot attempts to synchronize with her for the first time. During her activation test with Rei Ayanami, the Entry Plug is auto-ejected (apparently by the Eva herself) and Eva-00 continues to move undeterred until her power runs out. The second incident, involving Shinji Ikari, is extremely similar, except the Eva rejects the auto-eject signal. This prompts a cryptic comment from Ritsuko, "Is it going to try to absorb Shinji?" It is not clear, however, due to these circumstances if the two incidents of Eva 00 losing control should strictly speaking be considered true "berserker mode", and not simply Eva 00 "losing control". Case in point, Eva 00 turned off after its normal internal power supply ran out, which a "berserk" Eva would have been able to keep functioning beyond.
Eva-01 is the most notorious "berserker". Her abilities to move both independently and without power are effectively established before her initial activation, when she moves her arm to shield Shinji from falling lights (in spite of being loaded with a signal Termination Signal Plug). Not long after, she goes out of control during her first sortie, against Sachiel, exhibiting a bestial and uncompromising "battle rage" that doesn't stop until the Angel is down. Similar behavior occurs when Eva-01 brings about Leliel's gruesome destruction, and, later, mauls and devours Zeruel, although, unlike the incident with Sachiel, these both occur when her internal batteries are fully drained. In Episode 25', Eva-01 moves independently once more, setting off a crucifix-explosion and growing "wings of light". From this point forward, Eva-01 seems to be in complete control of herself.
Eva-02, after being ravaged by the Mass Production Evas, briefly goes berserk and moves without any power supply, moments before its destruction. This instance is unusual, in that Asuka is shown piloting, rather than control being clearly deferred to the resident soul (Kyoko), the only one who should be able to access that type of higher Evangelion ability (according to the precedent established with Eva-01). It is possible that this is because, as Asuka herself proclaimed, she by that point had fully realized that her mother's soul was inside of Eva 02, "watching over" her, and was thus able to integrate her actions with those of her mother's soul. Shinji, by comparison, did not fully comprehend that his mother's soul was contained in Eva 01 during the battles against Leliel and Zeruel, etc.
Eva-03 was infected by the Angel Bardiel during activation and initially behaved very much like Eva-01 in her first berserk incident, breaking open its jaws and bellowing, although this appears to have been the work of the Angel itself and not the resident soul. Despite being heavily mutated by the Angel, Eva-03 was ripped apart by a Dummy Plug controlled Eva-01, so the Angel appears to have been unable to harness the latent power and strength of a truly berserk Eva.
Notes
- Bousou literally means "out of control". "Berserk(er)", however, is the official English rendition, as indicated by the document Shiro Tokita displays in Episode 07.
- When controlled by the Dummy Plug system in Episode 18, Eva-01's behavior is indistinguishable from her true "berserker mode".
