From the moment we first get to know Wickham, we believe as Elizabeth prejudicially does, that he is a handsome, open, well-mannered young man, just what every young man ought to be.
When we first meet Wickham, we are given Elizabeth's first impressions of him, which are that he is extremely handsome, converses easily, and lacks arrogance and vanity, unlike Mr. Darcy. In every conversation he has with Elizabeth, he seems to be quite open and genuine, even though it is curious that he keeps bringing up Mr. Darcy. When he becomes engaged to Miss King and Elizabeth prepares for her journey to Kent to reunite with Charlotte, his farewell is "perfectly friendly"; he is even more friendly to her than she is to him. In fact, she tells her Aunt Gardiner that "he is, beyond all comparison, the most agreeable man [she] ever saw" (Vol. 2, Ch. 26).
Wickham does not really change as a character throughout the novel; rather, our perception of him changes along with Elizabeth's. We first learn Wickham's true nature when Darcy pours out his heart in an effort to defend himself against her ill-judgements of him. Elizabeth knows that even Darcy wouldn't invent a story that slandered his own sister's honor, and, therefore, feels obliged to believe that she had misjudged Wickham. Through Darcy, we learn that Wickham is deceitful, a squanderer, and capable of dishonoring others for his own personal gain. Wickham tried to seduce Darcy's sister for the sake of gaining her fortune and seduced Lydia for even lesser reasons. As Mrs. Gardiner reveals in a letter to Elizabeth, Wickham had never asked Lydia to elope with, had never intended to marry her, but left the regiment due to debts, and Lydia, out of mistaken understanding, decided to follow him, as we see in Mrs. Gardiner's lines, "[marriage] had never been [Wickham's] design ... [he] lay all the ill-consequences of Lydia's flight on her own folly alone" (Vol. 3, Ch. 52).
Hence, we see that not only is Wickham an immoral seducer, he remains constant throughout the novel; it is only our perceptions of him that change.