A lawyer once said to me, "A contract is only as good as the people who sign it." In other words, if a person doesn’t have integrity, their signature—their word—means nothing because they aren’t trustworthy. And if someone is dishonest in one area of their life, it's likely they will be dishonest in other areas of their life—including relationships.
Years ago, I went on a first date with a man at a lovely romantic restaurant. He ordered red wine for us, and no more than five minutes afterward, he started yelling at the waiter, demanding that we be served our wine.
I wouldn't call that date a failure or a waste of time because I learned something valuable about that person. I learned that we were not compatible within those first few minutes of the date. Someone’s character is quickly disclosed by how they treat others—which is how they will treat you, too. If someone is kind to others, they'll likely be kind to you. If they have integrity in their interactions with others, they will have it with others.