For one her predecessors were HEY LISTEN and the king of red lions who is basically absent at every point of the game that matters except at key plot points, at which he also shows himself to just be the king waiting for link to finish up the main plot rather than an actual companion.
So the expectations towards that game's sidekick were already pretty low, and boy did she overcome them. Starts off fake-evil, then slowly across the game reveals more and more, she's shown in an entire range of emotions from smug, to various degrees of concerned or worried, to thankful, to angry, to very angry, to whatever you call it when you're slowly dying, to content and in the end, sad. It really gives you the impression that she connects with Link.
I don't know man, TP fucked me up. No game made me this sad about it's ending.
I was all the more disappointed in Fi. I remember that whenever I was playing it and was sure no-one else was around, when she'd pop up again every 5 seconds to tell me the battery was low, I'd say, and once or twice almost shout at the screen to tell her something about how she's not Midna and never will be. No matter how hard she tries.