self-appointed 
adjective
Having assumed a position or role without the endorsement of others: self-appointed experts...
- It would be good to know if this is how he came across during his famous attempts to moderate American behaviour, in his self-appointed role as your best foreign friend.
- The film is dubbed a ‘social satire’ on America's self-appointed role as the world's police force.
- Their self-appointed role in life seems to be to treat their spouses as though they were small, mildly irritating but endearing children.
