Who is the main character in Fate/stay night Unlimited Blade Works? 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 poorly adapts 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.

Sources:


Ufotable's Type-Moon Wikia Page
Studio Deen's Wikipedia Page