You mean on the end of bisys, right?
It's an adjective here, because it modifies a noun, rather than standing as a noun in its own right.
Adjectives agree in gender, case and number with the noun they modify.
[The quote is from the top of the page linked to below.]
Solar nouns (which themselves often end in -s in the nominative singular) often cause -ys to appear on the end of class 1 adjectives (of which bisa is a subclass) for the nominative case (and the vocative).
I think bisa follows the class 1 adjective pattern when it is before the noun: https://wiki.dothraki.org/High_Valyrian_Adjectives
And its own subclass pattern when it is after the noun: https://wiki.dothraki.org/High_Valyrian_Adjectives#Bisa-type
But for further confirmation see also: https://wiki.dothraki.org/High_Valyrian_Pronouns#Demonstrative_and_interrogative_pronouns