I guess there might be people who don't know about this feature here...
It was known that there was something called
Bond Units, since the game's launch, but it was only last month that western fans really understood how this feature works. Basically, by raising bonds with other players (by visiting their castle, giving accessories, and registering their player card), when the bond becomes high enough, the two player avatars' bond breathes life into the land, giving birth to what is essentially a child unit of the two avatars - and, yes, male x male and female x female pairs can have children like this too, unlike with the S ranking supports.
This unit has a random starting class, and inherits the secondary class of each avatar, alongside dark prince. They inherit stats and skills from the avatars like a standard child unit. The actual unit is randomly chosen from a list out of possible children, which depends on the campaigns being used (so, Nohr x Norh has different possible results from Nohr x Hoshido). You can rename them, unlike Kanna. Their hair color seems to work like a standard child unit too, although it's random in the case of same gender pairs.
Their appearance was created using the avatar system, but they have a mark on their faces, similar to a brand, to differentiate them from the avatars. One of their possible portraits also is unique among the playable cast (a hooded man). Their personal skill depends on the actual unit that's randomly selected, many recycle personal skills from other characters, but some start with class skills as their personal skills, even Astra and Luna are there.
These units have no supports though, so they can't expand beyond their initial class set, aside from DLC classes, can't gain support bonuses (which are less important here than in Awakening) or interact with the other characters.
This is a shot taken using the fan translation patch, it's not mine:
Serenes has the full list of the possible Bond Units and their skills, alongside more gameplay information:
http://serenesforest.net/fire-emblem-fates/hoshidan-characters/bond-units/