Best-Behavior Friend
Also known as 'BBF'; a friend whom you have very little in common with and you act on your best-behavior when you're with.

A best-behavior friend does not typically know the extent of your true character or transgressions because you misrepresent the truth to make yourself look good or innocent. A person with a best-behavior friendship may see the friendship as important or long-standing and so lying about situations or leaving out key facts becomes common.
After a night of dancing, Meg found herself secretly having drunk sex with her best friend's ex-boyfriend. Meg went for lunch the next day with her best-behavior friend Rachel and when Meg recounted her night to Rachel, the sexual elements were removed.