Well, I haven't played the games Petey Piranha was in, so I can't say much, what I can say is that he was in one or two games as a boss and a few random (and I mean like marioparty or mariokart) games, but because Nintendo seem to like him for some reason or they other, he got into as a boss.
Impa? Well, I'm sure she's appeared in several games, but, she's only been... just there, at least in OOT, and in the handhelds I've played, she only seemed there at the beginning. She doesn't appear much else than Zelda's nanny and her character doesn't go much further than that.
Tingle? Well, if Petey Piranha got in as a boss I can definitely see him as a PC, he's appeared in way more than three games, and in several games he appeared and made his way reappearing in the game several times where you had to interact with him. In his games you get to see how he became who he is, why he is like that and how he lives his life. You don't do much with Impa in Ages and Seasons, only see her screw up at the beginning and talk to her a few times, and she does much less in ooc. Plus she hasn't appeared since then. Tingle? He makes you your maps, he translates them so they won't be completely useless, he tells you exactly where to go in WW. If I remember correctly you also had to speak to him several times in Ages? (I haven't played that in so long) and if you want to complete the miniquests in Minish Cap you have to talk to him and his bros almost constantly.
Malon? Yes, she gives you your horse in oot! But does she appear in EVERY other game, if not all? No!
I'd love to see Midna in brawl! Honestly I'd rather play her than Tingle, I love female characters like her! But I'm a realist here, and I think that after all his reappearances Tingle deserves the spot more.
Great, now I feel like a total noob. Sheesh.