Where does “ولی” come from?

ولی (Old Anatolian Turkish) comes from Classical Persian ولی, from Arabic وَلِيّ — near; custodian, protecter, helper.

ولی (Old Anatolian Turkish): one who displays a lot of religious virtues, and therefore is thought to have reached a supreme religious degree and/or God's love, and is recognized as holy; saint, wali