Bond Units are a gameplay mechanic introduced in Fire Emblem Fates.
When giving an accessory to another player at their My Castle, after having done so multiple times before, and having also received at least one accessory in return from that same player, a unit will be created. This unit is the "child" of the two players, resembling an Corrin that looks similar to the two who created it, such as having the same hair color or face type, distinguished by a mark on their cheek. They are not an actual child, as they can be created by two players of the same gender and the game announces them as being "born from the earth" due to the strength of the players' bond.
They will start as a level 1 base unit in the secondary class of one of their "parents" and have the last skill equipped on each (except for DLC skills), as well as having a randomized personal skill depending on which versions each "parent" is playing (see below). Their stat growths reflect the boon and bane of their "parents". They are able to reclass to Nohr Prince/Princess and a secondary class, usually the other "parent's" secondary class, but this is not always true. If both "parents" have the same secondary class, the Bond Unit's other class will be random, although there is a rare chance that either or both classes will be random anyway. Be wary though, as their starting class is not included in their reclassing options, so they cannot return to that class if reclassed, and they can only access one of their base class's promotions.
These units are only given to one of the "parents", the one giving the accessories, and can be renamed upon receiving. The unit will appear in the unit logbook under the "Bond Units" tab. They have the same restrictions as Einherjar, Prisoner and Amiibo units, such as not affecting the story or having any character supports.
Unlike other character's personal skills, the bond unit's personal skill is not unique to them. Whatever personal skill the bond unit receives is entirely dependent on the path each "parent" is playing through at the time of their creation.
If the "parents" are playing through the same path, such as both playing Birthright or Conquest, then the bond unit has a chance to receive the personal skill of path locked first generation unit, such as Odin's Aching Blood or Hinata's Triple Threat, or a (usually) path locked skill, like Astra or Gamble. If both parents are playing though different paths, however, the bond unit will have the chance receive the skill of a neutral character, like Mozu's Forager or Kaze's Miraculous Save, or a random skill from either path, like Rend Heaven or Demoiselle.
If either "parent" is playing through Revelation, they have a chance to receive the personal skill of a path locked child unit or special character dependent on the non-Revelation players current path. If both "parents" are playing through Revelation the unit will have the chance receive Azura's Healing Descant, Shigure's Perfect Pitch, or a completely random class skill.
The possible skills for each set are listed below.
Birthright & Birthright
Birthright & Conquest
Conquest & Conquest
Birthright & Revelation
Conquest & Revelation
Revelation & Revelation
When a bond unit is created, it is given a name dependent on the path the players are playing through. If both players are playing through Birthright, the Bond Unit will a Hoshidan name, while both players playing through Conquest will yield a Nohrian named bond unit. If the players are playing through different paths, the Bond Unit will have a name from either country.