Credits: FE Fates Wiki (general information)
“Bond units” are guest units created when two players form a “special bond” during My Castle visits.
Right now, information is limited, but it seems both players must gift an Accessory and register profile cards before a Bond Unit will appear. When successful, a brief cutscene will occur to signal the Bond Unit’s creation.
Bond Units look the same as Avatars you can create, but they have a brand on their face to distinguish them. On some occasions, they may use the “Hooded Man” face instead.
Essentially, Bond Units function like a “child” between the two players’ Avatars. (This is just a metaphor though and you can form Bond Units between same-gender Avatars).
Similarly, Bond Units inherit the last equipped skill from both Avatars.
Note: Because of a possible oversight, a Bond Unit’s primary class set is always Nohr Prince/Princess, not their initial class (unless their initial class is Nohr Prince/Princess).
As such, you cannot use a Heart Seal to return to their initial class or, if promoted, access their initial class’s promoted classes.
Available Bond Units
The Bond Units you can receive are randomly selected from a limited pool–and the types of units depend on the combination of both players’ campaigns.
Note: During creation of a Bond Unit, you may change their name.
3rd route x 3rd route
3rd route x Hoshido
3rd route x Nohr