I was thinking of examples such as 'boQwI'' or 'SuvwI''. I did mistakenly remember it as a plural suffix when it appears it is used to mean "one who does", similar to how Spanish uses "ero".
Even so. How is it used as "one who does" and "my" in this example?
There is a verb suffix -wI' meaning "one who does" or "thing which does", similar to English -er (a "digger" might be a person who digs or a tool that digs, for example).
There is a noun suffix -wI' meaning "my" which is used when the "possession" is capable of using language (e.g. "my father" or "my friend" but not "my dog" or "my rock").
The two happen to look the same -- so you have to know whether the stem is a verb (as with boQ "to assist") or a noun (as with SID "a patient").
A bit like how the English ending -er is also used to form the comparative of adjectives -- "stronger" is not someone who strongs or a thing that strongs, and a "digger" is not something that is more dig than something else.
You can tell the difference by seeing whether the -er is attached to an adjective or a verb.