Having searched on Google, I think I've established that a "Joker" is the specific term used for spies throughout the animé series, Kiba.
I found this paragraph:
"Very soon after, however, Cloud is attacked by a rather angry Despara, who he holds at bay for the second time. After Despara, who at the time became Jesara, gets killed after being struck by Morocco's blade that had been intended for Roya, Morocco attempts to flee. Cloud however makes quick work of Morocco by delivering a fatal stab through his stomach, effectively killing him. After Roya and Zed return to Templer, Cloud sits by Jesara's side for the last time, because she had been sharing the same body with the now-dead Despara. Afterwards, Cloud returns to Tusk to continue his ways as a
Joker."
All through the
web page, "Joker" has a capital "J", suggesting that it was the author's idea to call all the spies "Jokers". At the beginning, it says that the series is set in a fictional world where characters are separated into factions and races. It appears the author simply chose to call one of those factions "The Jokers", and they are spies.