Is Tidus the main character of Final Fantasy X? I think many would say he is. I would have said that when I first played the game. He is the first character the player meets, the story is told through his eyes, and at the beginning of the game, it seems like everything important in the story is happening to Tidus. He is thrust a thousand years into the future. His father is Sin. He can't get home. Tidus even says, "This is my story." Upon closer examination, however, I've come to the conclusion that Tidus is not the main character of Final Fantasy X; he is simply the narrator. He is a narrator who plays an important part, yes, but Final Fantasy X is not Tidus' story. It's Yuna's. Although Tidus says, "This is my story," he proceeds to tell us the story of Yuna and her pilgrimage to defeat Sin.
A protagonist is defined as the leading character, hero, or heroine of a drama or other literary work. Yuna is the hero of Final Fantasy X. She is the one who gives the people of Spira hope. She is the one on a journey to defeat Sin. All of the antagonists in the story are against Yuna, not Tidus. It's clear that Final Fantasy X is her story, although later in the game she says, "This is our story."
Having a major, but not main character narrate a story is not a new technique. A major example of this is the Sherlock Holmes stories, where the narrator is Sherlock's best friend, Dr. Watson, and not the protagonist, Sherlock Holmes. Square-Enix even does it again in Final Fantasy XII, where Vaan is the character players see the story through, but is not the protagonist. This technique can be used for many different reasons.
In Final Fantasy X, I think Tidus being selected as the character we see Yuna's story through was incredibly smart and well thought out. Tidus makes the perfect narrator for many reasons. The first and most important, in my opinion, is that he isn't from Spira. With the exception of blitzball, everything in Spira is foreign to Tidus. The player learns about Spira with Tidus. Because he doesn't know anything, the other characters explaining what should be very basic things seems natural. Another reason Tidus makes a great narrator is because he is so involved in the story. He has a history with Auron, he is Jecht's son, he is Yuna's love interest, he is like a brother to Wakka... Through Tidus, players are able to attain information about all of the characters and see all sides of Yuna's story. Many characters confide in Tidus and tell him information that he is told not to share with others.
Even when Tidus is taken out of the story, joining Auron and Jecht in the Farplane, Yuna's story continues. Something she says in a speech to the people of Spira after defeating Sin is super important.
The people and the friends that we have lost, or the dreams that have faded... Never forget them.
Although I don't think Tidus is the main character of Final Fantasy X, Tidus is still important to the story, and it wouldn't have been the same without him.