Question(Beware Spoilers): The REAL Last Guardian?

Question: WHO exactly is the last guardian?

(SPOILERS! You have been warned.)

So I've tried to google this but the search parameters don't help. I've watched Let's Plays of The Last Guardian and cannot help but wonder at the title. Logically you would think that the little boy is the mentioned "Last Guardian" of the game title. He's the main protagonist, frees Trico from being controlled, forms a bond with Trico and he's probably the last one to have those cool tattoos. However, thinking about the story, the boys were supposed to be turned into those magical barrels that the trico eat for energy meaning they weren't supposed to be "guardians" at all. The "Master of the Valley" who was mind?controlling all the tricos and golems is the one actually protecting tricos as a species, keeping a system running and making sure the tricos maintain their magical energy. Plus the boy killed him, thus being "the last".

In a way you could argue both the boy and the Master of the Valley could both be the last guardian but if we take it more literally, my stance would be that the title refers to the antagonist Master of the Valley as the real last guardian since he was destroyed. If the now grown up boy wanted to, it may be possible to train his heirs as future guardians so he wouldn't technically be the last guardian. That's my thought anyways.

Tell me your reasons for what you think!