The man Mel Gibson hired to manage Oksana Grigorieva's singing career claims she's a two-faced opportunist who was constantly "pushing [Mel's] buttons" ... this according to legal docs obtained by TMZ.
Michael Lustig submitted a declaration for Mel in the child custody case -- in which Lustig claims despite Mel's efforts, Oksana never showed an ounce of gratitude.
In the document, Lustig states, "In my experience, Ms. Grigorieva's disposition is generally anxious, depressed and uneasy, and she constantly 'digs' at Mr. Gibson and pushes his buttons."
Lustig adds, Oksana would become a "dark" person when she would argue with Mel ... noting he could often hear her yell at him ... and her words were "sharp, pointed and nasty."
As for little Lucia -- Lustig claims he once advised Oksana to cancel a trip to Moscow because of a terrorist attack in the city ... but she left Lucia behind and traveled anyway.
Oksana survived the trip. As for the custody case ....