There have been many questions as to the title of the game -- The Last Guardian. Who exactly is referred in these two words? Is it the boy or is it Trico?
Most people believe that it refers to the boy, since he was the last one who brought the ending to all the child kidnappings. As you know the children have been turned into the blue goop that functioned as a source of energy for the protectors of the valley -- the armored knights. So, one could say that the children have been turned into the Guardians of the Valley. Since the boy was the last child to be kidnapped -- he is the Last Guardian.
Another theory suggests that the title refers to Trico -- which also makes a lot of sense. Throughout the entire game Trico thoroughly serves and protects the boy from all enemies, apart from the times when it gets hypnotized by the Master of the Valley’s mechanisms. But other than that, Trico was the Last Guardian for the boy -- it helped him get to the main tower to destroy the Master and return back home safely.
Who knows, maybe the title refers to both of them, and in this sense both the boy and Trico serve as The Last Guardian for each other.