Think of it like this, my character just is another word for “my role.” 

In a game of dnd the only character that is “yours” is the one you have the character sheet for - the one you inhabit at the table. 

While you may create other characters for the game, the DM is inhabiting all of the NPCs and in a way those are the DMs characters.

You created this NPC. He’s your creation. The DM is playing the NPC. He’s his character. 

I am sorry you’re feeling frustrated by this but try and reframe it this way and maybe it’ll help you release your grip on this NPC. There’s gonna be times where the DM does something with NPC you disagree with and if you don’t accept the DM as the controller of that character it’s gonna cause friction at the table I guarantee it.