How is Roberto best useful to Viola?

Who was Roberto in twelfth night? I don't recall him.

Roberto is best useful to Viola as he guides her, and tells her about the twist and turns of the people and the surroundings in which she is going to live.In the very begning of the play Act 1 Scene 1 Roberto, the captain tells Viola that her brother is alive as he has seen him.He also introduces Viola to the land of Illyria and tells him about Duke Orsino who loves the lady Olivia and that she has lost her father and brother and living a mournful life.Viola finds Olivia alike and decides to serve her in order to hide herself from the world and for her survival but Roberto says that she'll not accept this as she doesn't like to meet any visitor.So, Viola decides that, in this case, she will disguise herself as a young man and seek service with Duke Orsino instead.Roberto agrees to give her a manly attire and does what so ever she wants from him.