The croupier's wife (SPOILERS)

It's highly ironic and kinda funny the fact that, when she was presented as a character on the film, she was the most innocent and clean (and less interesting) of all.
Being married to an ex-pimp, she knew nothing of his past, nor did she have any relation to crime.
Every single other character of the film was either connected to crime in any way (i.e. Marc or the lieutenant), or lived at night (like Yvonne or Anne).
Yet, every one of them had it's own code of ethics and knowledge of right and wrong. Even Marc, who wasn't the most likeable character, didn't talk to the cops because of his feelings towards Bob. He was forced to squeal, since he was afraid of going to jail.

What makes the wife's character so interesting is the reason of all her actions. Her motives. It would seem rather fair, if she informed the cops in order to keep her husband from being involved to a criminal act.
However, that was not the case.
She did yell at her husband when she found out about the heist, but for another reason. Because he should have agreed on a larger commission! How funny is that! And she - the classic, innocent, law-abiding housewife - literally took him by the hand, to go find Bob and demand a bigger share.
Only when they didn't manage to find him, did she decide to follow her plan B, which was letting the cops know of the heist.
Without any hesitation or regret no matter what.

It was the only "clean" character on the film, that eventually turned out to be someone with absolute lack of ethics or honor.