Shield

"A defensive Trigger. It's more durable the smaller it is."

- Osamu Mikumo about Shield

is the main Defense Trigger in Border. Its durability can be increased by activating and/or by focusing it.

Appearance
Shield generates a smooth, transparent green barrier whose shape and size can be freely altered. When it is activated, a glowing green hexagon appears on the user's hand or in correspondence of the body part it was cast from. The majority of Border users summon hexagonal Shields, possibly suggesting that is the Trigger's default shape. The barrier also seems to have a default size, allowing it to protect the head and torso of a person of average height, as it leaves a fleeting, luminous impression of its original size when compressed.

Overview
It is unknown when Border created this Trigger. Since some Neighbor countries use a similar Trigger, it may have been part of the technology that Border acquired in the Neighborhood. Its performance rate has increased over time.

Abilities
Shield is considered an indispensable Trigger for all Border agents, regardless of their rank or position. It allows the user to conjure a barrier to defend against enemy attacks. Since it is capable of hovering, it leaves the user's hands free. It can move along with the user or remain fixed where it was cast. It can generally be deployed anywhere within 25 meters of the user, although the distance oscillates according to the user's trion levels and individual ability. Shields can be cast remotely by one or more agents to protect an ally. An immobile barrier will be more durable than a moving one. A Shield which suffers more damage than it can withstand will crack and eventually break.

The durability of a barrier is inversely proportional to its surface area and directly proportional to the user's trion levels. With normal trion levels and bullet parameters, a Shield broad enough to protect the user's head and torso can withstand multiple rounds from a Gunner Trigger as well as Lightning. Conversely, it is necessary to erect a barrier measuring only a few inches across to block an Egret shot or a thrust with Scorpion, whereas a thrown, spinning one is capable of piercing it. Kō Murakami, one of Border's top Attackers, was able to crack one such focused Shield with a single swing of his Kogetsu, breaking through it moments later by continuing to apply pressure. The process of shrinking a Shield appears to be virtually instantaneous, and, aside from defensive purposes, it may be undergone to remove a barrier from the path of one's attack without dispelling it alogether. Should the user be unwilling to reduce the surface area of their barrier, they can muster enough defensive power to defend against Egret by activating a second Shield, provided they have enough chips in their Trigger Set; however, casting Shield through both the Main and the Sub, an act known as Full Guard, prevents the user from fighting back, so caution still needs to be exercised. Full Guard with unfocused Shields and a single, focused Shield can both be easily pierced with Ibis, unless the user boasts exceptionally high trion levels: Hyuse and Masataka Ninomiya are capable of repelling it with an unfocused Full Guard, while even one Shield cast by Chika Amatori can withstand the impact.

A Shield can not only be concentrated, but also fully shapeshifted, even after it has already been cast, or be divided into a myriad of small barriers. Gunners can create an opening in their Shield to fire their weapon without exposing themselves, its transparency allowing them not to lose sight of the enemy. Multiple agents may combine their morphed Shields to increase the retain a large surface area without sacrificing durability. Spherical Shields allows the user to protect themselves against multidirectional or explosive attacks without sacrificing motility.

Fixed Mode
Alternatively, they may resort to Fixed Mode, which increases its durability in return for the inability to move it from its original position. It is possible that, in order for this mode to be activated, the Shield must be given a prismatic shape, fully encasing the user, and/or rest on a solid surface at the moment of activation. Despite its large surface area, a fixed Shield is capable of withstanding the damage from most of a Shooter's Full Attack with a composite bullet combined with the explosion of a planted Meteor, vastly surpassing the defensive abilities of an ordinary, unfocused Shield. With normal trion levels, that same fixed Shield will be able to guard the user against Ibis when paired with a focused Shield.

Shield does not interfere with Triggers which lack direct destructive power, and is thus unable to block Lead Bullet.

Countries in the neighborhood
Combatants from Galopoula possess a very similar Trigger, which is pentagonal and seemingly more durable than Border's version, since it was capable of blocking Fūjin's remote slashes. Users either have at least two such Triggers in their equipment, or are able to generate two barriers with the same one.

Chelidon can generate circular barriers similar to Border's Shield, whose number and diameter can be increased or decreased at will.

Rhodokhroun, a planet-nation that specializes in the making of Trion Soldiers, are able to manufacture models that can generate shields, such as Manberdo Dogs and Idras. The latter are particularly remarkable for their ability to combine their shields to increase their durability, but even the shield of a single Idra can withstand a Kogetsu slash without cracking.

Users
Shield is part of the Trigger set of every A and B-rank agent participating in Rank Wars, as well as in Jin's, Shinoda and those of the members of Tamakoma First, for a total of 93 known users, which makes it the most popular Trigger in Border. The total number of Shield chips, however, is a minimum of 183, as all but six official agents have two Shields in their Trigger set; the exceptions are Kyōsuke Karasuma, Osamu Mikumo and the four Bagworm Tag users..

The only confirmed users of Fixed Mode are Ai Kitora, Chika Amatori and Daichi Tsutsumi.

Additionally multiple Neighbors have been shown to use Shield.

Trivia

 * The first Shield used in the series was in Fixed Mode.
 * When Chika used her Shield in Fixed Mode, a piece of the roof she had been standing on prior to the activation of the Trigger was shown to be encased inside of the barrier. It is unclear whether this means that Shield can phase through objects not made out of trion, that it automatically expands when obstacles are removed, or something else.
 * In Border, dummies can be fitted with Shields to help Shooters practice.
 * "Shield" is also the name of one of the seals of Yūma's Black Trigger. The two are also similar in appearance and functions, and both are more durable when fixed in place.