A Concealer is a type of Crosser that has an in-born ability to hide their Ether stream from other Ether-sensitive beings, making them undetectable to most untrained or poorly-trained Crossers, as well as Strays (who normally do not have enough depth of Ether strength to detect a Concealer's presence) and Panzers (whose Crosser-detection technology works on critically mistaken base assumptions).
The origins of Concealers are poorly researched, but older records show that they existed as early as during the height of Egyptian power in present-day Middle East, where it is said that mediums often communicated to, and spread the wisdom of the "cat deities". This remains a myth, as there has been no surviving document from Egypt detailing the existence of such Concealer mediums, or why only Concealers are allowed to even speak to cats, among other scientific inconsistencies.
Nevertheless, one confirmed appearance of Concealers are the Japanese ninjas - shinobi - speculated to have begun some five hundred years before the dawn of the Tokugawa shogunate. During a time when Ether was still a poorly-understood concept/power in Japan, the ninja - known for their jumping and fighting prowess - were the forerunners, able to execute their nefarious missions with deadly accuracy, presumably with a little help from their Ether abilities.
However, for reasons that are equally poorly understood, the shinobis disappeared with the arrival of the Tokugawa Shogunate, presumably to avoid persecution; speculation is rife on the matter among Panzer scholars.
Why are Concealers so hard to detect?Edit
Please refer to: Ether Detection
Known traits of Concealers and reasons for their disappearanceEdit
Concealers are only extensively understood as being synchronous with that of shinobi-do, hence most Panzer researchers have asserted that Concealers have an innate talent for quick movement and the martial arts. In modern day terms, that means Concealers are potential parkour experts and close-up bruisers, and/or masters of melee weaponry such as swords.
That is also understood to mean that the kunai-throwing scenes, made famous in 20th century period TV dramas, were really Enomena in action acting against physical objects.
However, Crossers come in all sizes and shapes, and hence there is no telling what other abilities Concealers may be capable of, just as in the case of many Crossers, who may inherit abilities through training, as well as possess unique abilities by chance.
Concealers are also understood to have a smaller capacity of Ether than regular Crossers; this is a proven fact with solid evidences, such as well-known shinobi clans never sending clan members for warrior training, yet this is paradoxically coupled with extremely poor understanding (given the rarity and obscurity of known Concealer families); due to poor knowledge of Ether workings, no reason is formally given for the lowered Ether capacity.
It is also argued that the Concealers (at least, those in Japan) may have died out because, bruising as they are, the shinobi houses steadfastly refused to collude with wealthy Crosser families.