Originally developed as an independent, deep space exploration platform, IS is also capable of atmospheric flight.
The fundamental technologies applied to IS are completely different from those of conventional techs, making it a potentially deadly weapon.
In fact, IS is so powerful compared to conventional weapons that it could render any current military platform obsolete.

Primary protection for IS and its pilot is provided through use of energy shield (invisible to naked eye) that could block both physical and optical weapons.

The protection level of IS shield is sufficient to be impenetrable to most conventional weaponry, but it could be penetrated by high-powered IS weaponry.

The basic armor plates of IS offer minimal protection against incoming attack, since any weapon powerful enough to pierce the shield would be also powerful enough to penetrate the armor plate.
(That should explains why the pilots are so scantly covered by armor plates)

However, special equipments such as additional physical/energy shield or augmented heavy armor plate (specially designed to withstand damage) could provides moderate to adequate level of protection.

The integrity of energy shield is maintained unless the IS is depleted of power, but the shield tends to lose its strength when nearing a complete power depletion.
(Same thing applies to all other components such as armor plates. Weapons, especially, tend to have difficulty maintaining the materialized shape when nearing power depletion)

Weapons powerful enough to penetrate the shield is capable of damaging IS or injuring the pilot directly.

IS is also equipped with an ability called Absolute Protection.
(or Absolute Barrier.. the official English term for this is not introduced yet)
This ability protects the pilot from an attack that could cause critical injury or even death to pilot.

This ability is said to be able to protect the pilot from any kind of attack, but it also has seriously high power consumption, depleting the power of IS with just single activation.
(In that sense, it is more like pilot safety mechanism rather than combat defense mechanism)

Another advantage for IS is the use of PIC (Passive Inertia Control) technology.

By incorporating PIC, IS could accomplish extreme agility and mobility unparalleled by any conventional platform.

PIC largely absorbs and negates impact shock and protects pilot and IS safe from catastrophic event such as high-speed collision.

PIC is also applied to flight assist, and coupled with gravity control technology, makes IS capable of flight and hovering.

Advanced sensor suit package called Hyper Sensor is another prominent feature of IS.

When the pilot is linked to Hyper Sensor, she receive augmented sensory input data from IS's sensor, directly into 5 sensory system in her body.

Simply put, when piloting an IS, the pilot's senses such as vision are greatly improved.

Using the sensory data input from IS, the pilot gains 360 degree field of view, being able to 'see' directions that are not possible for human body without turning her head.
(Such as behind, directly above and below)

However, because such improved sensory input is much different from normal human body, the pilot must process the input date into something she could comprehend first.
(The speed and efficiency of processing such data decides the effectiveness of IS piloting)

The major difference of IS from hardware metaphor is that all components of IS are stored in the core as data, and are called up (materialize) when needed.

In Closed Form, IS automatically begins recharging its power and repairing any damage it received without requiring outside power source.

It is possible to call up IS by parts; such as calling up arm component only (to block incoming attack with that arm) or thrusters (to gain increased mobility or gain flight capability)

Partial call-up requires some training, but generally it is much faster than calling up the whole IS.

Since the parts of body not covered by IS activation are just vulnerable as normal human body, partial call-up should be used with care.

An IS core is the essential part that comprise nucleus of IS.

If you compare the composition of IS to a PC, IS core would be CPU + power supply. In other words, IS can't be activated without a core.

Currently, the technology to create an IS core is not known to the world, and there are maximum of 467 active IS at any given time.

(There is only one person who could create IS core - the scientist who originally created IS - and that person disappeared after completion of 467th core)

The IS core is more than just a machine parts, since it contains some kind of AI-like intellect, able to evolve by itself

The core could be removed from IS frame and transplanted to another IS frame.

Many countries use this feature to upgrade their IS force to the latest spec as peripheral technologies for IS devices are developed.

Since it is not possible to mass produce IS, the R&D for increasing individual performance of IS and training talented, highly skilled pilots are main focus of new arms race of IS era.

Although 10 years have passed since the introduction of IS, the technology regarding the production of IS core remains a mystery.

Other components of IS, however, went through rapid technological advancement and numerous generation changes occurred during past 10 years.

The IS is generally categorized as having 4 generations.

1st Gen. IS : The primary focus is completion of operational IS as a weapon.
2nd Gen. IS : The primary focus is multi-role platform with use of various Equalizers.
3rd Gen. IS : The primary focus is primary anti-IS platform with use of newly developed Image Interface technology and advanced weapons derived from it.

As the story begins, 2nd Gen. IS is nearing its final phase of service, with newly developed 3rd Gen. Prototypes beginning to undergo field tests.

Since there are so few active ISs in the world, it is virtually impossible to personally 'own' an IS.

The ownership of IS is limited to national government or international organization with similar size or importance.

However, there are few pilots who are assigned IS for prolonged use.

These pilots are called Designated Pilots, and the IS assigned to them are called personal ISs.
(Since the actual ownership of IS is not transferred to the pilot, it is not a personal IS in a true sense)

One example of Designated Pilot would be the National Representative Pilots.

The reason for having Designated Pilot and Personal IS lies with the very nature of IS itself.

IS possesses self evolving mechanism, and this ability is manifested only when IS is kept paired with a pilot for a prolonged time.

This is due to the nature of IS trying to optimize its ability and efficiency with its pilot.

When change of pilot occurs for some reason, the IS resets the progress and revert to default setting in order to begin a new optimization process for new pilot.
(Think of it as formatting a HDD for every new user)

Once a pilot is registered to an IS, it begins a process called 'Fitting' which is initial phase of optimizing it for that pilot.

This process is also called '1st Shift,' and various adjustment are made in IS, including its exterior shape.

When the Fitting process is complete, the IS gains a new form, which is called '1st Form.'

This is a phase where IS is roughly optimized for the pilot. Still, in this state, the IS operates at much higher efficiency and gains increase in overall performance.

If the IS gains enough experience while operating with same pilot, it eventually reaches another phase shift, this time called '2nd Shift.'

The IS is optimized even further by this process, and the resulting '2nd Form' is considered as a goal for any Designated Pilot.

Personal IS in Closed Form is stored in a form of an accessory worn by the pilot all the time.

In that way, it could be called up whenever needed.

Non Personal ISs are stored differently. In this case, IS retains the physical manifestation of its components even when inactivated, and call-up just activates IS without any significant visible change.

Many ISs that are used for training purpose have this feature disabled since the nature of trainer IS - meaning many different pilots operate it - renders the time consumed by Fitting process and its effectiveness completely useless.

They are also difficult to transport, because they retain their shape and weight even when inactivated.

When IS receive damage, it automatically tries to repair the damage during its Closed Form period.

It is possible to activate a damaged IS (as long as it has power left) but such action - especially entering combat situation with damaged IS - is highly undesirable, since IS would try to cope with the given situation only with components that are still operational.

As IS gains experience during any situation while it is active, (be it normal training or combat, or even a simple checkup) it records data about its every components and proceeds with optimization process.

Activating a partially damaged IS causes the inoperable components (damaged parts) to stop optimization until it is fully repaired.

For an example, If an IS loses a leg, and is forced into combat it would try to maintain its performance as best as it could without use of a leg.

The resulting data would be implemented throughout its component to improve their efficiency, but the damaged leg would be left out of equation.

So, even when the leg is fully repaired, it would show a discrepancy of efficiency compared to other components, which will continue to compound the situation as the IS keep on operating.

This kind of situation could have serious implication since it would interfere with general optimization process and could even prevent the IS from reaching 2nd Shift.

For that reason, even the most hot-headed pilot tends to refrain from forcing damaged IS into action.

If a Designated Pilot is found out to be preventing her IS from evolving further, she would most likely be replaced with a new pilot, and as honorable as becoming a Designated Pilot could be, getting its position revoked is even more disgraceful - something every Designated Pilot tries to prevent at all cost.

Equalizer is the name for any kind of add-on equipment for IS.

All IS comes with a basic weaponry, called Preset, but it also has a number of expansion slots that could be used to field additional equipment.

Equipment comes in various forms, such as additional weaponry, thrusters, and augmented armor plates.

These Equalizers need to be installed into IS first.

Since every IS core is unique and each of them has its individuality, (one might say personality) it is possible for a IS to reject installation of certain Equalizer.

The Equalizers could be assembled into package type for an IS, designed to fulfill a specific task, as long as there are enough expansion slots left.
(A bombardment package would consist of something like following sets of Equalizers : Long-range cannon + Stabilizer device + Augmented armor plate)

Equalizer Package tends to be designed for a specific IS, since components such as additional thrusters or armor plates would have to fit the IS.
(For the same reason, hand-held weapon type Equalizers tend to be more compatible with other IS)

Equalizers that are attached to IS (such as thrusters or armor plates) are called up simultaneously with IS, but hand-held weapon type could be called up while in activation.

In this way, the pilot doesn't have to worry about carrying numerous equipments during activation. She just needs to call up the weapon that is needed at any given time.

Equalizer, just like an IS in closed form, is called up by pilot's thought process.

It is possible to call up by announcing the name of Equalizer, (or IS) but this is only because the announcing act helps the pilot to concentrate her thought (which is the actual command)

The 'announcing' way is considered as a method for inexperienced pilot who has difficulty with call-up just with mental command.

Also, verbally announcing the Equalizer or IS basically ends up warning the opponents, and verbal announcement takes rather long time compared to the time required by experienced pilot to mentally command call-up, which takes fraction of seconds.

Since Equalizer is materialization of installed data within IS (This is same for IS) even if it is damaged or destroyed during combat, it is not truly 'gone,' but the it does require maintenance or restoration just like damaged IS, and is rendered ineffective or inoperable during those period.

A day before the Silverior Gospel incident, Chihuyu calls all 5 heroines to her room and ask them about the reason for being in love with Ichika.

This is also the first time Chihuyu shows her true self to them.
(a care-free, alcohol-loving ordinary woman side which even Laura did not know about before)

After listening to all the answers from the girls, she begins to brag about all the good things that happens when having him around. Some of them are;
- He is a great cook.
- He is great at housework.
(He used to do all the house chore because Chihuyu was always out working to make living for both)
- He is an excellent masseur (Cecilia gets to experience just how good he is...)

After all these bragging, The dialogue between them goes like this.


=============
Chihuyu : So, do you want him?
Girls : Huh?
Laura : Um... are you giving him to me?
Chihuyu: Who said that I'd give him away? You fool.
Girls : (WTF...)
Chihuyu : If you are a woman, become strong enough to take him by force. So train hard, kids.
=============

Well, that's what happened in Vol.3.