I just want to know who is the main character in Fate/stay night Unlimited Blade Works, in the sense that every season has a different main character.
[...] every season has a different main character
This is actually incorrect. Fate/stay night always has the same main character Shirou Emiya. In the visual novels, you always play/read in the perspective of Shirou, with some exceptions:
Various interludes (such as in the Fate route, the scene where Saber left to attack Ryuudou Temple in the middle of the night, going against the group's decision not to attack, is played in the perspective of Saber).
The Prologue, which plays at the beginning when you start the game up for the first time (replayable from the Extra's Menu). You play as Rin Tohsaka from the morning before she summoned Archer until the point where Archer was attacked by Saber.
However, depending on the route, there is a second character whom the story focuses on. The female main character is always the heroine/romance interest of Shirou during the said route.
- In the Fate route, it is Saber
- In the Unlimited Blade Works route, it is Rin Tohsaka
- In the Heaven's Feel route, it is Sakura Matou
So in regards to the Fate/stay night anime adaptations, Shirou is always the male lead, while the female lead in
Studio Deen's 2006 Fate/stay night anime adaption, it is Saber (the majority of the series adapts the Fate route)
Studio Deen's 2010 Fate/stay night: Unlimited Blade Works movie adaptation, it is Rin Tohsaka (the movie
poorlyadapts the Unlimited Blade Works route)
Ufotable's 2014-2015 Fate/stay night: Unlimited Blade Works anime adaptation, it is Rin Tohsaka (it adapts and adds onto the Unlimited Blade Works route)
Ufotable's TBA Fate/Stay Night: Heaven's Feel anime adaptation, it is Sakura Matou
Apart from the heroines, their Servant also get featured in their respective routes:
- In Fate, it remains as Saber
- In Unlimited Blade Works, it is Archer
- In Heaven's Feel, it is Rider
I also remembering reading somewhere that there was originally a plan for a route in which Ilya was a Heroine, but it was later scrapped and merged into the Fate Route (apparently, a dinner scene in which Ilya is a part of was taken from this route). So had this route been published, Berserker most likely would be the Servant featured.
So to answer your question, Shirou is still the main male character, but Rin Tohsaka is the main female character in Fate/stay night: Unlimited Blade Works.
According to the author of the game (Nasu) it would be Shirou in all 3 routes and the main theme of the game is his growth and being able to conquer himself.
When asked about the main theme in an interview Nasu said:
Interviewer: - What was the theme you are trying to express?
Nasu: The main theme is "conquering oneself." There are three story lines in Fate, each has a different themes. The first one is the "oneself as an ideal." The second one is "struggling with oneself as an ideal." The third one is "the friction with real and ideal." This game is describing the growth of the MAIN CHARACTER Emiya Shirou. The first storyline shows his slanted mind, the next storyline shows his resolve, and the last storyline gives another resolution for him as a human. All three storylines are essentially equal, but they have different forms.
And there the part that answers your question the most is:
Nasu: This game is describing the growth of the MAIN CHARACTER Emiya Shirou. The first storyline shows his slanted mind, the next storyline shows his resolve, and the last storyline gives another resolution for him as a human.