A Warder (known as Gaidin in the Old Tongue) is a person, traditionally male, who is bonded by an Aes Sedai through the use of saidar to become her bodyguard and protector. Most Aes Sedai bond one Warder, but the Green Ajah allows the bonding of multiple Warders and the Red Ajah, until recently, did not bond Warders at all; in addition Browns and Whites often do not bother with Warders unless it is necessary for their research. Aes Sedai sometimes marry their Warders, but this practice is most common among the Greens. Warders are easily recognized by their distinctive color-shifting cloak, made of fancloth, when they choose to wear it.
The Warder bond has distinct benefits for both parties. The Warder gains greater stamina and physical prowess, a greater capacity to resist evil, the ability to sense shadowspawn at a distance, and greater resistance to injury (totally aside from the proximity of his Aes Sedai, who is probably able to Heal his wounds using the Power). The Aes Sedai, for her part, gains a bodyguard, confidant and ally-in-schemes who is intrinsically linked to her and can be compelled, to a certain extent, through the bond. Both parties are able to sense the other's general location, physical well-being and, to some extent, emotional state.
When the bond with a Warder is broken he rarely survives. He completely loses the will to live and will almost always die attempting to avenge his Aes Sedai. The breaking of the bond is also terrible for Aes Sedai, creating so much emotional turmoil for them that often Aes Sedai take many years to bond another.
Whether by custom or law, all Warders are male; the only woman known to currently be bonded as an Aes Sedai's Warder is Birgitte Trahelion, bonded to Elayne Trakand, who did so because Birgitte was dying for reasons that standard Healing could not address. Also, Aes Sedai are capable of bonding several Warders at once, though only the Green Ajah have been known to actually do this; generally, they are bonded one at a time, as opposed to several through the same bond, and it is unknown if Warders bonded this way have any extrasensory awareness of each other. It is also unknown if multiple Warders can be held via the same bond, as opposed to the current method of one per Warder, though three women (Elayne Trakand, Elmindreda Farshaw and Aviendha of the Nine Valleys sept of the Tardaad Aiel) have recently bonded a single, communal Warder (their mutual lover, Rand al'Thor), so it is possible the reverse could be done. Women who cannot channel (Min Farshaw) can hold Warder bonds, though obviously a channeler is necessary to create the bond between them. It is possible for two separate women to bond the same man as Warder on separate occasions, without interfering with each others' bonds; al'Thor, at the time of his bonding to his three loves, had already been bonded (against his will) by Alanna Mosvani in a fruitless attempt to control him. It is possible for an Aes Sedai to relinquish her Warder bond and pass it to another Aes Sedai, such as when Moiraine Damodred passed her bond with Lan Mandragoran to Myrelle Berengari. Finally, Warder bonds can be dissolved unilaterally by the Aes Sedai at any time.
As of the later books in the series, Asha'man have discovered their own form of the Warder bond using saidin and have begun bonding women as Warders. In the beginning, married men bonded their wives, primarily for the benefit of being able to "touch base" with her through the bond, but recently Asha'man have also bonded a small number of captured Aes Sedai, much to the consternation of the White Tower. This form of the Warder bond permits the use of Compulsion to control the captive Aes Sedai. In addition, five Asha'man (Damer Flinn, Jahar Narishma, Karldin Manfor, Eben Hopwil, and Naeff) have been bonded (consensually) as Warders by Aes Sedai. In Knife of Dreams, Mazrim Taim grants permission to Red Ajah sisters to bond his Asha'man. Additionally, Rand al'Thor grants forty-seven of the Salidar Aes Sedai to bond any Soldier or Dedicated they wish (to balance the fifty-one sisters bonded by Asha'man). The fact that both parties can channel has had no (known) effect on the Warder bond, but there has been speculation that male channelers universally cannot be controlled the way normal Warders can.