What about your heart?

[Sephiroth]

Actually, wasn't the Wikipedia article but the KH Wiki

Everything that is a "Wiki" is the same for me. All of it can be "updated" by everyone.

I think saying that strength is qualitative, not quantitative, exactly alludes to what you said.

Possibly yes.

Moreover, it could be argued that all of such factors that you mentioned could be considered, or even productive of, a strong will. You can easily say that Sora has a strong will because of his friends and desire to help them, Xehanort will is strong because he wants to rule Kingdom Hearts, etc...

The problem is that willpower is its own kind of "spiritual energy". It all works together, right, but I cannot say it is the same. If I equate will and heart, especially in Kingdom Hearts many things get messed up, even if Ansem still has Xehanort's will and consciousness while he is his darkened heart. Correlation is right and fitting as long as you don't go too far making it all the same.

Saying that the antagonist also has a strong heart is a bit jarring, than saying he has a strong will.

Especially in Kingdom Hearts I would say "strong heart is legit". Heart is such an important word and a will itself is pretty much the most important thing a person has. in Kingdom Hearts it seems a bit different, yes, especially because all of it is very similiar but to make a good example: Xehanort is known to have a very powerful heart - Sephiroth is known to have an indestructable and incontrollable (except himself) will, the most powerful of all. I would never use those terms too equal because "heart" is also very often used for "soul" and it is also, especially for Kingdom Hearts, used for "the essence which contains the emotions as well". I always equate soul, consciousness and will in their meaning with the essence so it would be okay to do this with the heart as well but when it comes to comparing will and heart there are differences like the will which is never used as "container of emotions". Even more, normally it is what is inside the heart and mind what makes your will stronger. It can be from both sides, yes, but I always notice how much more (and more important) comes from the one direction, so what makes your will stronger (in life, not Kingdom Hearts). Your heart is pretty much growing automatically and does not necessarily need actions which are a consequence of your will, but the will really getting stronger always requires more especially because the will getting stronger is not just about "I am strong for the ones I love" but often requires a certain strength of the will itself in the first place because you need to say to yourself "stop doing this crap and get your ass up". I get philosophical so let's stop. If I can tell you something, I can also see this "strong heart -> strong will" in Kingdom Hearts better than "strong will -> strong heart". Like in real life.