Bond Supports are fixed supports between certain pairs of characters that usually grant 5-10% bonuses to Critical and Critical Evade while they are directly adjacent. They only appear in Monshō no Nazo, Thracia 776, Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn (and to an extent, Fire Emblem: Shadow Dragon).
Unlike normal supports, Bond Supports do not involve unlocking Support conversations initially, or require any building of support points between the characters in order to actually activate the bonuses. Instead, they are already present in gameplay by default to indicate the pair share a close relationship (usually by being kin or intimate friends). The player is able to add more bond supports for characters by having them reach an A support with another character in Path of Radiance, and then transferring the data to a Radiant Dawn save file where it will manifest as a 5% bonus (unless the characters already share a default Bond Support, in which case the bonuses will not stack). The exception to this is Largo since he is no longer playable in Radiant Dawn, so his A supports cannot transfer. All bond supports are not one-way and provide the bonuses to both characters, provided they are next to each other.
Enemy characters are not excluded from having bond supports, and several bosses possess a few, such as Shiharam with Jill, Pain and Agony with each other, Dheginsea with Kurthnaga, and Sephiran with Sanaki or Zelgius. Despite this, however, enemy units cannot utilize their Bond Supports unless the character they share the bond with is of the same affiliation as them.
In the in game description of the game in Fire Emblem Radiant Dawn says that the unit can be within three spaces of each other. This however is a typo as it is still the units have to be adjacent.