Neon Genesis Evangelion's most frightening feature is not actually the deadly force attacking Earth. Instead, that which strikes the most fear and poses the most questions are the mecha themselves. For the manga, the artist reports in one of his interviews that he was approached and asked to draw a kind of metallic demon. If the enemy is called an Angel, that makes sense. However, as the series progresses, even when violent and out of control the Evas pass beyond the realm of simple 'demons' and become something else. Why say this? The Evas rip and tear and destroy, they were designed to look demonic, and yet they shouldn't be considered demons? The easiest way to analyze this is to break the Eva's design and characteristics down systematically. Their appearances, their behavior and their reactions to both their human pilots and their Angel counterparts are all to be examined.
Let us begin on appearance and potential gender of the Evangelion. Evas themselves seem rather androgynous, lanky and skeletal. Unlike many other mech suits in anime design, they lack bulging muscles or particularly evident armor. Perhaps for this reason, when looking at an Eva, they are do not come across as masculine. They even appear to have a thin waist and hips. In this way, they are much more feminine. One of the scientists in the series refers to unit 01 as 'she' on several occasions in the dubbed anime. In the series it is explained that the Eva 01 actually 'absorbed' a human woman (Shinji's mother) and so this may be the reason for that reference, but there are other things which support the labeling of an Evangelion as female. When pilots enter the Eva they are submerged in a life-sustaining formula which Shinji describes as warm, and smells like blood. Pilots are then brought inside the Eva, where they are protected, much like being re-immersed into a womb. Without their pilots, the Evas are capable of far more damage and dangerous feats. It is almost as if when there is a pilot to be wary of, the Eva is more careful.
Asuka refers to her Eva as 'mother' a few times during her insanity. This is more support in favor of the female/mother theory. While Shinji's mother was absorbed by an Evangelion, Asuka's mother had never been near an Eva. In fact, there was very little to support the fact that 02 was actually related to Asuka's mother in any way. The Evas themselves, however, seem to have a kind of maternal instinct in terms of the protection and care of their chosen pilot. If they can, when their pilot is injured, they will destroy any threat in the area regardless of what that threat may be. Unit 00 even attempts to kill Gendo Ikari at one point. In this way Evas are far more dangerous than Angels- on multiple occasions the Evas have gone berzerk and fought or attacked in a very base and carnal way. It is as if the Eva is a de-evolved version of the Angels, bent on the protection of their pilot child, and NERV has not yet come to domesticate it.
As is seen early in the series, an Evangelion itself is a living being covered in armor. It is revealed much later that they are actually clones of Lillith or Adam which have been severely altered. In this way they are related to humans as well as Angels. Like Angels, they are capable of great power, but like humans, that power often comes from feelings of pain or emotion. To control this and limit it, the Eva's jaws are 'locked' with a metal hinge. While this hinge is not explained in the series in any detail, I suspect its effect is similar to keeping a wild animal muzzled. If the animal never had a chance to use its jaws, it would have no idea what it was capable of. The jaw locks may keep the Evas subdued out of ignorance. In effect, the lock hobbles an Eva's willpower so it will more readily accept a pilot as the dominating force of will.
The pilot's location inside an Eva is not as one would normally expect from a mech anime. The most common place mech pilots sit is the head, as if they were a brain; or the chest, as if they were wearing a gigantic suit of armor. Instead, Eva pilots are inserted into what could only be called the spinal chord. Pilots of an Eva are not so much a pilot as they are a secondary brain- a superego, restricting what the Eva will do rather than determining. As mentioned before, when pilots are removed from control, the Eva will berzerk out of control. Dummy plugs, or 'false' pilot signals, are not so much a 'pilot' as another circuit along the Eva's nervous system- they just allow the signals to continue passing to the Eva's brain. When a dummy plug is activated inside an Eva, the Eva rampages out of control much like cases where the pilot is absorbed or falls unconscious. Without a conscious mind to monitor the Eva's behavior, they will fall back on what their instincts tell them to do.
The flipside of this is what is called in the series 'hyperflux', or when a pilot begins to feel the pain of the Eva as it is injured. A pilot going into hyperflux has become too much a part of the Eva's 'nervous system' in this case- they're possibly thinking too similarly to the Eva itself and so begin to feel the pain when they should not be able to. This seems to happen most often when the pilots are focusing on emotions such as fear or jealousy rather than the cool logic their 'superego' role asks of them. This is possibly what causes pilots to be absorbed into the Evangelion as Shinji's mother was. If the Evangelion and the pilot are working too much in unison- the pilot 'agrees' to whatever the Eva is 'thinking'- the pilot becomes more a part of the Eva and less a safety breaker. When pilots cease to think differently from the Eva they may become a part of the Eva itself.
In contrast, End of Evangelion the movie showed that hyperflux can be taken to another extreme. If a pilot's will is strong enough to overrule the will of the Eva as a creature, the pilot can become even more in control of the Eva. Shinji, when his will is strong enough, not only feels the pain of the Eva but is able to perceive what it perceives as if it were his own body. In this way, it is almost as if the mother figure of the Eva finally relinquished protecting its child, seeing that child was now strong enough to fight on their own.
The skills of a pilot reflect the capabilities of the Eva as well. Evangelion are never shown using ungainly bizarre weaponry like many other mechs. Evangelion weapons are very simple and crude. A large knife, a large spear and an oversized hand grenade are all good examples. For the Evangelion to be capable of something more requires training of the pilot. To defeat an Angel using a gun, Shinji had to be trained in the use of fire-arms. When fighting under the control of pilots, Evangelion are not often seen doing such things as flips or cartwheels. They run, dodge clumsily, fall down and fight as a human being would. This again sharply contrasts an out-of-control Evangelion with its restrained and piloted counterpart. Out-of-control Evangelion are bound by no such limitations- they can flip, run, regain balance and fight with extreme skill. The fighting style, however, is nothing like that of a human pilot's. Out-of-control Evas are animalistic and almost always resort to ripping and tearing their enemy apart. Sometimes they are even seen crouching on all fours, and they frequently roar. Although they are genetically similar to a human being, without a pilot they are bound by no human decency or self doubt. This is different from the behavior of Angels who are similarly related to humans.
Angels do not cause wanton damage in Evangelion. When they invade a city, they have a very single-minded goal which is to reach NERV's headquarters and connect with Lillith/Adam. They waste no time in getting to work on this goal. Angels, unlike the monsters in Power Rangers or Godzilla, don't spend their time crushing Tokyo underfoot and throwing buildings around. Their reaction to Evangelion trying to stop them is more defense than offense. They do, however, show primal emotions such as fear. Kaoru shows an even wider range of emotions, but even he is never distracted from his goal.
The relationship between Evangelion and Angels is odd. Evangelion seem to have an instinctive hatred of anything that threatens their pilots, and so will readily kill an Angel. At the same time they seem far more complacent to follow the instructions of any given Angel than those of any given human. Tabris (Kaoru) was able to synche perfectly with all of the Evas, instantly. Unit 03 (Touji's Evangelion) was taken over by an Angel in a matter of minutes, with no fight back on the Evangelion's part. Unit 02 didn't even need to be entered for Tabris to control her. It seems almost that if a pilot the Eva doesn't approve of is inside (Touji, Asuka going insane, etc.) or there's no pilot at all, they have no qualms with following an Angel. Evangelion, in a way, are the trained attack dogs of Earth. When they're told to they will fight viciously, but otherwise they don't seem all that maddened.
In the end, even at the finish of the series, the viewer is left with more questions about Evangelion than answers. Although, if one watches for repeated actions and imagery, these patterns of behavior and reaction will be evident. The Evangelion themselves are the demented, berserker form of parental affection which are torn between their human creators and their animalistic nature. They express a will of their own, although it can be suppressed by their child pilots. By the finish, Eva are some form of alien mother. Their use and reactions stem from this.