How would you describe each of the main characters of the Book of Matthew? What were your first impressions of the protagonist and antagonist?

The Gospel according to Matthew is not a literary work with a conventional protagonist and antagonist, but rather a biography of Jesus, a holy man. This makes the Bible part of a distinct genre following its own distinct conventions. Some of the characters you might want to discuss are below.
Jesus: The protagonist of the work is obviously Jesus, who is the central character, present in every chapter, and held up as a model for emulation and worship. You might actually want to use "Christ" as an adjective because it functions as a descriptive term meaning "anointed one" (its Greek meaning) or Messiah (Hebrew equivalent). Another key descriptive term is "Son of Man," which occurs more frequently in Matthew than in the other Gospels. 
John the Baptist: This important figure was described as a forerunner of Jesus in Matthew 11:1-19, and is portrayed as an austere prophet who lives in the wilderness.
Satan: Matthew 4:1-11 describes Jesus being tested by Satan in the wilderness. The devil can be considered an antagonist to Jesus and is evil and duplicitous by nature. 
Mary: The Virgin Mary, mother of Jesus, is an important positive character in the work, both as the woman who bore Jesus and in her mourning for him after his death.