Vessel

A vessel (依代 Yorishiro) is the partner of a summoner, who will be possessed by the Material called in by the summoner during the summoning ceremony.

Principles
Modern summoners work in pairs with a vessel. They use the Blood Sign ceremony to summon Materials by carving out their names and having them possess their vessel. While vessels are usually other humans, this isn't a rule and animals can be used as vessels, like Aika's white liger. Unlike summoners, one needs inborn talent to be a vessel. An excellent vessel can be considered a type of genius.

Those with the talent to be a vessel normally don't become a summoner. This is because vessels act as a contact point for Materials. If vessels tried to be a summoner themselves, the Material would hijack their body instead. The very fact that a pair is needed for the summoning ceremony proves that the vessel needs the support of a third party to maintain control. Because they are a contact point for Materials, vessels always wear symbols of bondage to prevent a vengeful or evil spirit that has not been summoned from taking control of their mental state from the outside. These symbols of bondage take many forms, from simple collars and handcuffs to inconspicuous things like a wristwatch. According to Kyousuke Shiroyama, the symbol can be anything, as long as it’s a symbol that solidifies the vessel's heart; something the vessel uses to restrain their actions or desires. Kyousuke has also mentioned some common symbols for girl vessels, such as a scale, their skirt’s zipper, etc... The symbol apparently needs to be modified and handed over to the vessel by the summoner to guarantee its effectiveness.

Not even a vessel can fully control the Material that possesses them during a summoner battle. The Materials themselves have certain desires that drive them: the desire to devour flesh, to suck blood, to crush everything in sight, to smash everything after petrifying it, etc. The vessel cannot stop those desires. All they can do is try to control the aim by choosing who those desires will be directed at. A Material will prioritize counterattacking whoever attacks it. It does not matter whether or not that attack did any damage. It is possible for the vessel’s will to correct the targeting cursor, but the Material still takes an unnecessary action.

Imaginary Flashback (憑空暴走（イマジナリーフラッシュバック） (Imajinarī Furasshubakku)) is a condition that can happen in vessels. Any frustrations, desires or pains held just before transforming into a Material will return with twice the intensity when they transform back. It is like having your forgotten sleepiness come rushing back once the effects of the energy drink wear off.