Ice Climbers were the main characters of their game... and play a major role, being the main protagonist. Just like Mario.
Not sure if there's a difference here.
Also, Ganondorf was actually more background than anything. He was about as notable as Bowser in the first Super Mario Bros. Yeah... he took over the world, yada yada.
We knew he was cunning, right?
But he was the main villain.
Also, as said, established is a pretty bad way to look at it. It doesn't apply to Mother, really. It barely had two games, and only the second game was acknowledged.
I explained Ganondorf.
Lucas had a single appearance as well, and wasn't even supposed to be the main character!
Ice Climbers, main character.
Also, let's look at Yoshi. A few appearances at best. Maybe 5 at most. Super Mario World, SMW 2, Yoshi's Story, Mario Party 1 and 2. Oh, wait, 6. Mario is Missing.(could've been other spinoffs) In only two games was he "important", if you will.
Sheik played a decent role in OOT. About as much as Ganondorf, to be honest.
Lastly, we have... Captain Falcon. He's about as important as Ice Climbers, save having more characterization.(what, people forgot he was a one-off too?)