In my mind, I unconsciously have always been putting every characters in movies, games or animes into 4 different boxes: Main hero, main heroine, main villain, Secondary characters. Secondary characters are the rest of the characters that are not the main hero, main heroine or main villain.
Whether among the secondary characters you'll get to separate them again into "most important secondary character" or "least important one", I don't care. But that's the fact and that is that. No matter how much you love the secondary character or how much you hate the main heroine, don't throw your shit and claim that your secondary character is main heroine.
Maybe that's because I've always been watching anime, drama or playing games ever since I was little. But that's how it goes.
Good evening to my darling main hero of FF7 Cloud Strife and main heroine Aerith Gainsborough, and screw you to the hell and back the main villain of FF7 Sephiroth.
(And yes, that means anyone other than the three of them are secondary characters!)