In the first season only four of ajin(s) are assigned to four characters in the series. Though lots of ajins are seen but not seen next to their human counterpart.
So far in the first season. There's only one female ajin.
Ajin is the name given to the humans who can't die. The humans can regenerate any part of the body including the head but might lose memories if decapitated
Ajin(s) can only last 10 minutes when summoned
Throughout Season 1 the title of every episode is spoken at some point in the respective episode.