Unfortunately, you asked more than one question, so I had to edit some out. Mr. Wickham seems to be quite an upright gentleman in Pride and Prejudice by Jane Austen, at least at first. Though Elizabeth Bennet is an intelligent young lady, she is fooled by his deceptive appearance and manners as well as her own prejudices.
His appearance was greatly in his favour; he [Wickham] had all the best part of beauty, a fine countenance, a good figure, and very pleasing address. The introduction was followed up on his side by a happy readiness of conversation—a readiness at the same time perfectly correct and unassuming.
In fact, Elizabeth is so moved by Wickham's appearance and manners that, the first time she meets him, she says he is far superior to every other man in the room. It is no wonder, then, that she believes him when he tells her a story designed to make himself look like a martyr and a victim of the cruel Mr. Darcy.
His story is that he had always intended to be a minister rather than a soldier (which he now is); however, he was unable to pursue his passion for the ministry because he does not have the money he was promised by Darcy's father, money which was intended to help provide for his needs but which he does not have. According to Wickham, Darcy kept the money by finding by a devious maneuvering of his father's will.
This story is not particularly believable to me for several reasons. First, it is a story designed to get pity and/or place Darcy in an unfavorable light. A man who wanted to enter the ministry would not be talking like this, and neither should a soldier. Both are honorable professions, and what he says amounts to gossip, whether it is true or not.
Second, Wickham tells the story so easily and quickly to a virtual stranger, which is what Elizabeth is to him. He is well aware that he is a dashing and rather persuasive fellow, so this story seems somehow contrived to me.
Finally, every story has two sides. Anyone who is telling a story to get sympathy is only telling one side of the story. Elizabeth knows these things (as she demonstrates in her distrust of Mr. Darcy) but is swayed by Wickham's persuasiveness and her own ready willingness to believe the worst about Mr. Darcy.