Unbreakable Spirit. The first time we hear this in in Skyward Sword.
It is stated that it is necessary to use the Triforce properly, along that such an individual must also grow spriritually in Power, Wisdom and Courage in order to be balanced so that the Triforce does not break into three fragments,

My question is of which persons we can say with certain, that they actually possessed such a spirit.
It also begs the question, if those with the Spirit of the Hero (all other Links) have all an unbreakable spirit as well.

We know that SS Link has one and it is implied that Hylia/SS Zelda possesses one as well as she wanted to use the Triforce herself.
AlttP Link, AoL Link, OoT Ganondorf and King Daphnes Nohansen Hyrule were able to wish on it, but it is uncertain if they had an unbreakable spirit in the first place.

Interresting to note from gossip I have heard is, that the japanese version of Skyward Sword uses Spirit of a Beast instead of Unbreakable Spirit. It still needs to be comfirmed though.

If it is indeed true, then it would indicate that OoT Link (Hero's shade) and TP Link have those, as they appear as beasts in Twilight Princess.
AlttP Link appears as a bunny however in the Dark Realm.

Any further clues, dear colleagues?