Duke or Count of Illyria named Orsino: 

Orsino is the emperor of Illyria, despite the play's contention over his title, and he has some prior maritime experience. He is infatuated with Lady Olivia but she won't reciprocate his feelings; Viola, who is hired as the young Cesario, swiftly gains his trust and tries to seduce Olivia on his behalf. 

By the play's conclusion, though, Orsino's sentiments have changed; he no longer engages in self-indulgent behavior and has grown fond of Viola.

Viola:

After losing her twin brother Sebastian and becoming stranded, a young woman decides to make the most of her position and enter Orsino's service as a young eunuch by taking the name Cesario. Viola is given the task of courting Olivia, despite Olivia falling in love with her as Cesario. Orsino rapidly wins over Viola's trust and confides all of his heart's secrets to her. 
Although she is unable to confess it, Viola is in love with Orsino. Despite this, she is successful in winning Orsino's adoration and changing his character to one that is more honest and less selfish.

Curio, Valentine.

Two of Orsino's staff members. Valentine is dispatched to woo Olivia, but he has less success getting inside Olivia's home than "Cesario" does. 
In the last act, Curio goes with Orsino to visit Olivia, but he stays silent the entire time. Curio's primary duty is to take care of Orsino, but they are not as close to him or as significant to the drama as Viola is.

Second Officer, First Officer:

These two identify Antonio as having wronged Orsino, and they then place him under arrest. As they remind Orsino of the crime that Antonio had committed in the final act, they are holding Antonio.

A sea captain:

He saves Viola from drowning and assists her in becoming Cesario so that she might serve as Orsino's page. Later, he is meant to have some sort of conflict with Malvolio, and Viola needs to track him down so she can get back her clothes and belongings.

Sebastian:

Brother of Viola who is not her identical twin but looks very much like her. Since the two are frequently confused for one another in the situations, Viola tries to imitate her brother while posing as Cesario. Antonio saves Sebastian, and the two hit it off right away. 
Sebastian stumbles upon Olivia and mistakes her for Cesario; she pops the question, and despite Sebastian's ignorance of her identity, they get married quite shortly.

Antonio:

Sebastian's life is saved by another ship captain, who also seems to have Sebastian's best interests at heart. But because of his dubious past and involvement in some maritime misdeeds, Orsino is looking for him. 
Erroneously believing Viola to be her brother, Sebastian, he interrupts combat between Sir Andrew and Viola. Although this can be debated, it has been suggested that his connection with Sebastian is a little more than amicable.

Olivia:

The Countess of Illyria is an aristocrat. To frustrate Orsino's attempts to court her, she decides to stay in mourning for her brother for seven years after losing both her father and brother shortly before the play begins. She falls in love with Viola, a courier for Orsino, and despite Olivia's best efforts to win Viola over, Viola makes every effort to let her down gracefully because Viola is not a male as she falsely represents herself to be. 
Thought to be Viola as Cesario, Olivia hurries off and marries Sebastian, but all turns out okay in the end.

Maria:

The clever, cunning gentlewoman-maid who works for Maria Olivia and harbors some feelings for Sir Toby. She decides to get even with Malvolio when he humiliates the group. 
She pretends to have Olivia's handwriting and plans to utilize that talent to deceive Malvolio. She is genuinely a pleasant person who can hold her own with the "boys"—Sir Toby, Sir Andrew, and Feste—despite her love of a good joke.

Sir Toby:

Belch is an unfortunate yet realistic last name. He is Olivia's misbehaving uncle, and he enjoys drinking, getting wasted, and having fun with Sir Andrew. He participates in the tricks played on Malvolio and also plans the combat between Sir Andrew and Viola. He has a nasty temper and is typically boisterous, but he's not a bad guy either.

Sir Andrew:

The surname Aguecheek, while not flattering, accurately describes his thin, pale features. He is an absolute moron who cannot but misinterpret everything Sir Toby says. Even though they don't interact much in the play, he wants to win Olivia's love. He challenges the young man to a duel, which is never nearly completed, out of jealousy at the attention Olivia gives to "Cesario."


Malvolio:

The steward of Olivia is a man who is severe and doesn't enjoy having fun while having a good time at work. He gives Sir Toby and the others an extremely hard reprimand, and they decide to exact retribution. By convincing him that Olivia loves him and making him look silly in front of her, they prey on his conceit and pride. 
He is eventually imprisoned and tortured by Feste; in the final act, he comes forward and recounts his ordeal, vowing vengeance on all parties involved because he hasn't learned any lessons.

Fabian:

a mysterious family member who lives in Olivia's home and has undefined duties. He joins Sir Toby and Sir Andrew in the plots against Malvolio, acting as a sort of mid-play replacement for Feste. 
Although he doesn't appear to have any personality, he is necessary for the play's exposition and is adept at pointing out the obvious to the audience.

Feste:

a musician and jester who helps Olivia and Orsino both. Feste is witty and adept at wordplay, but as evidenced by his treatment of Malvolio, he can also be fairly harsh. 
Feste may play the fool, but in reality, he is highly clever and insightful about people's personalities; he knows the ideal music for any occasion, but he dislikes it when people take advantage of him.

A priest:

Sebastian and Olivia are wed by a priest, who later confirms their union to Orsino and others.