You can't tell me that Cale doesn't fit the stereotypes associated with foxes, cunning and beautiful, a trickster and a con man, someone wise and strategic who can turn anything around. ( ‾́ ◡ ‾́ )
Cale has all of those characteristics and although I have seen some fics where he is a member of the fox tribe I have never found one where he is a nine-tailed fox in this case, a kumiho.
Here comes my ideas:
Kim Rok Soo's parents were kumihos who decided to live hidden from society and therefore hid their true race from Rok Soo with plans to tell him later.
But the car accident they had when Rok Soo was eight ended up killing them before they could tell him about his inheritance.
Rok Soo grew up knowing nothing about his blood and heritage until the cataclysm where in a fit of hysterical adrenaline he manages to break the disguise of him later portraying him as an ability.
His life in the shelter differs greatly from canon because of this since he has an ability, but the curse of the white star ruins everything again.
His life continues as in canon until the moment of transmigration where he wakes up with Ron seeing him in a confused way and even without knowing him he interprets it as a cold look acting in the same way as in canon.
By some miracle he manages not to see himself in a mirror until he enters the bathroom where he screams in surprise.
Then Ron appears behind him saying "From your reaction I assume you don't know what this is, do you young master?"
Now he has to deal with his "initial ability" transmigrating with him, hiding his fox traits, and creating a convincing lie.
(Later during the Sloth Test: "You mean to tell me this was never a skill?!")