Throughout 1st John there is a common theme of love. John covers everything from loving each other, to loving God, and the sin of loving this world. Although 2nd and 3rd John does not specifically share the theme of love, it is still evident in his letters to the various churches that he loves and cares about them.
The way John writes his books and the style also varies. Where as John focuses on writing the last two books in the forms of short letters, he writes the first book the way he did with the Gospel of John. His purpose was more to teach believers. The last two books had teaching in it, but I felt like he was connecting more with certain people and churches. However he still uses his talent of teaching in the last two books.
Although the way John wrote his books may be different, his characteristics stay the same for the most part. John was a very godly man and his writings reflect that. I believe that John was blessed with a deep understanding of love. As he grew older and followed Christ until the very end I believe his understanding for love deepened. Though he may show it differently throughout his books his characteristics definitely show through. Characteristics such as love, confidence, and holiness are present. I think that is very common in all of his writings.
Out of the five books of the New Testament that John wrote, his personality is evident in them. As he grew though and became more spiritually mature you can see his similarities and differences.