You have a group of people working together. Their morals aren't particularly important, but they won't be paragons of virtue. They may be hardened criminals, or they may just be pragmatic and look out for themselves first. But there is another person in the group who holds themselves to a higher moral code. Again, the degree varies. For the pragmatists, this person may indeed be a paragon of virtue. For the career criminals, it's more likely to simply be someone that puts a higher value on human life. But just by their very presence, the other members of the group gradually start to tone down their immoral behaviour. Gradually, they start to act more "good". The effect may well be slow, and may well be small, but it's noticeable. Named for the effect a well-played Paladin can have on an adventuring party in D&D.
Example: Rock to the Black Lagoon