Simba, a young lion cub, is the main character in The Lion King. He is born at the beginning of the movie, and throughout grows and learns, showing his journey in the circle of life. Simba is born a price, his father being King Mufasa. Because of Simba's birth, he is next in line to be king, putting his uncle Scar second. This makes his uncle Scar angry and causes him to wish his brother and nephew dead. In this way, Simba could be considered a factor in his father's death.
Mufasa is the king of the Pride Lands, and also Simba's father. Mufasa is scar's older brother, however, they are very different. In this movie, Scar would represent dark and evil, whereas Mufasa would represent bright and good. Musfasa only wants the best for his son. He teaches him skills, tries to keep him out of dangerous places such as the elephant graveyard, and, in the end, gives his life to save Simba. In a religious respect, Mufasa represents God.
Scar represents evil in The Lion King. As a lion, Scar does not seem to be involved with his family. Part of this might be his bitterness towards not being king, and now being second in line. Mufasa devises a plan to kill Mufasa and Simba and make himself king, which for a period of time, seemed to have worked. Scar lives in the darker parts of the Pride Lands. When Scar is "king", the entire land turns from bright and happy to dark and sad. In a religious respect, Scar represents the devil.
Timon and Pumba are Simba's friends which he meets after he flees from the Pride Lands. While I wouldn't consider them major characters in the movie, Timon and Pumba become very close with Simba and come to love him. It is Simon and Pumba that learn Simba's story and convince him that he needs to go back to the Pride Lands and take back his land.
Timon and Pumba are to Simba in the movie, as Zazoo is to Mufasa.
Rafiki shows up in The Lion King in some very specific parts. He is a very significant, although he is not seen as much as other characters. Rafiki represents the medicine man, Shaman, or the Priest. Rafiki's job was to baptize the young cub Simba with his blessing, and look after him in years to come.