The eNotes "Character" section on Edmund Spenser's The Faerie Queene defines exactly what each character represents. The Red Cross Knight (or Redcrosse) represents Saint George and "the Christian man's search for holiness." The Dragon represents evil and sin. Arthur represents magnificence (or the "perfection of all virtues"); and Una represents "truth and the church."
Another site, which supports "The Project Gutenberg," defines each of the characters even further. This site breaks down each character by 1) the moral the character represents; 2) the religious or spiritual aspects or associations of the character; and 3) the personal and political aspects or associations of the character.
According to the Gutenberg site, here are the breakdowns of the characters in question.
Redcrosse/Red Cross: holiness (moral), reformed England (religious/spiritual), and St. George (personal/political.
Dragon: sin (moral), Devil/Satan (religious/spiritual), and Rome and Spain (personal/political).
Arthur: magnificence/private virtue (moral), Protestantism (religious/spiritual), and Lord Leicester (personal/political).
Una: truth (moral) and true religion (religious/spiritual)--no personal/political associations were made.