A close friend of the protagonist is keeping a secret. Actually, they are keeping many secrets. None of which the protagonist knows anything about.
First, they are cheating on their spouse, with the mayor. To be fair, their spouse is a terrible person but the scandal could make the national news and that would be very bad for this friend.
Second, the friend is the one who stole the protagonist’s car. They sold the car for cash to a known trader in stolen cars. They feel guilty about it but felt they had no choce.
Third, they were once a member of the mafia and have been in a witness protection program. Several powerful criminals are serving long prison sentences. Only, they have all been found guilty of things they did not do. This friend committed all of those crimes.
Forth, they anonymously reported the trader in stolen cars to the police giving them evidence enough to finally make an arrest.
Fifth, and this is the thing they feel the most shame for, they short-changed a local racist. They’ve been doing it for years. This has resulted in a large unreported income that is hidden in a bank account in the cayman islands
They sound like a terrible person but – with the exception of the cheating – everything they have done was to protect their two teenage children. They had thought that the dangerous part of their life was over but an enforcer from the crime group they betrayed has recently arrived in town.
This prompt is a plot twist. Plot twists (a bit like writing dares) are ideas to throw a spanner in the works for your characters.