Sun symbolism surrounds Tidus. From his Celestial Weapon, the Caladbolg, to his name, I think that Tidus is meant to represent the sun in Final Fantasy X.
Tidus' name, pronounced Ti-da in the Japanese version of the game, comes from the Japanese Okinawan dialect, and means "sun." It comes from Tidanukwa or Tedako in Japanese, honorifics used to refer to a king, meaning, "child of the sun." I think that Tidus was meant to contrast Yuna, and his name is appropriate because Yuna's name means "night."
Yuna, in particular, with the Moon Crest and Moon Sigil required to activate her Celestial Weapon, is probably meant to represent the moon. For Tidus to activate the Caladbolg, he has to acquire the Sun Sigil and Sun Crest. Tidus is absolutely the sun to Yuna's moon.
Yuna's story is a very dark one, and her perceived future in the story is bleak. Preparing to sacrifice herself to rid Spira of Sin for a few years, Yuna is very serious and would probably be very quiet without Tidus around. Tidus inspired her to laugh and to have fun with her life. After laughing with Tidus, she tells the others that she wants her journey to be full of laughter. In this way, Tidus brings light to Yuna's life. He also made her question Yevon, and her own existence in the world.
In a lot of cultures, the sun represents knowledge. Tidus is the narrator of Yuna's story, the character the player controls. Cleverly, he knows as little as the player knows, and learns about Spira with us. He constantly questions what he sees in Spira. He is the one to realize that things aren't quite right with Yevon, and he makes the others see it, too. Like the sun, Tidus shines light on the world.
Now, I'm about to get a little poetic. I've been thinking too much, so bear with me.
The sun has to set for the moon to shine. Surrounded in darkness, Yuna is the light for the people of Spira. She is their hope. For Yuna to defeat Sin forever, she had to lose Tidus. Tidus had to sacrifice his life so that Yuna could go on shining. Although gone, Tidus continues to be Yuna's source of light. She holds onto him and cherishes his memory. She radiates off of him.
Although Yuna is her people's hope and the light that they see, Tidus was their source of life. Without his sacrifice, Yuna would not have defeated Sin. The sun, in general, is the source of life for all living creatures.