Gonzalo is an older counselor who is faithful to Antonio. He is a person of good character. When Antonio ousted Prospero as Duke of Milan and decreed he and Miranda be set out to sea in a boat, Gonzalo was put in charge of sending them off. He was thoughtful enough to make sure Prospero was supplied with books and other items that were important to him beyond the mere necessities of life. As Prospero explains:
By Providence divine.
Some food we had, and some fresh water, that
A noble Neapolitan, Gonzalo,
Out of his charity, being then appointed
Master of this design, did give us, with
Rich garments, linens, stuffs, and necessaries,
Which since have steaded much. So, of his
Knowing I loved my books, he furnished me
From mine own library with volumes that
I prize above my dukedom.
Prospero says that the supplies Gonzalo provided have helped ("steaded") him greatly and also praises Gonzalo's "gentleness." Gonzalo shows himself during the play to be a compassionate and hopeful person.
Sebastian, Alonso's brother, is another story. Just as Antonio usurped his brother's throne, so Sebastian wants to take Alonso's place as King of Naples. Sebastian is an evil individual with a low-minded, unforgiving nature. He is easily influenced by the evil Antonio into hoping to murder his brother so that he can become king.