In any given generation, there are always two Queens, who are female twins born from one of the last Queens, and they are naturally antagonistic with one another. Saya and Diva are the current Queens of their species. They were discovered by Joel Goldschmidt in the 19th century inside a mysterious creature found in Iceland known as SAYA. In his experiment, he raised Saya like a daughter, but kept Diva locked away in a tower with virtually no human contact, or even a companion to keep her sanity.
Unlike all other chiropterans, Queens appear to retain a human appearance at all times. They may also have some shape-shifting abilities, though during the series their abilities seem to be limited to modifying to their outward human appearance.
Queens spend only a few years awake at any given time, between decades of hibernation within a cocoon. During the hibernation process the bodies of the Queens form a sort of fleshy covering around their bodies similar to the cocoons that they are born in. This is probably used to protect the Queens during their hibernation cycles and to keep their scent from being detected by others of their kind to avoid being fed upon. There is only one known way to awaken a Queen, which is either forcibly or naturally and that is through the blood of her chevaliers, presumably the ones that had been first converted. It is known that forcibly awakening a Queen is an extremely dangerous and suicidal process resulting the Queen going on a berserk state and killing everyone in sight whether be it friend or foe. It's possible that until a queen creates a chevalier the queen doesn't need to go into hibernation, this is shown with Saya and Diva being awake for over 50 years before each of their first hibernations, during that period no chevaliers we created until around the time of Joel's birthday party massacre. Only after that happened did either twin go into their hibernation cycle.
The strength of a queen's traits are dependent on their feeding regularly on human blood. As such, Diva, who freely feeds on humans, can also freely use all of her abilities, while Saya, who avoids feeding, is usually only slightly above human level strength.
Queens have enhanced speed, strength, and endurance that surpasses those of their chevaliers and other chiropterans. All other chiropterans are born of the blood of Queens, either through the queen's own efforts or human experimentation. Their blood can also crystallize and destroy any chiropteran of any level that was created by their sister queen. Queens are believed to be able to defeat their chevaliers due to their enhanced abilities as shown by Diva when she defeated and nearly killed Solomon after a short time with him having grave injuries while she sustained none whatsoever from the engagement.
When a Queen becomes pregnant with the next generation of Queens, her blood will lose its dangerous qualities. A queen can only be impregnated by a chevalier created by her sister queen, ensuring the purity of the species is maintained. But there seems to be a specific part of the reproduction cycle in which a Queen will either feed off of a person so they are turned into a chevalier in order to survive or sense the other Queen's chevaliers and the one that she wants to reproduce with. As well due to the Queen being the originals of the race, the chevalier cannot do anything against her while the process begins with her eyes glowing their respective color to paralyze him through a blood reaction leaving him defenseless.
Procedure of Death
The twin Queens are the only dangers for each other as each of them can die by the others blood. This method of death is only viable if neither Queen has become impregnanted, a process that would cause the blood to no longer become lethal to the other Queen. As well the possibility of taking out the heart completely from the body and crushing it or beheading could be an effective method as well but due to the Queen's high level of fighting skills this becomes exceedingly difficult as the two tend to be equally matched in all areas of ability.