The significance of appearances: Ansem and Xemnas

Many fans treat Ansem as though he was nothing more than a mere extension of Xehanort, and Xemnas as though he were Terra's Nobody.

I'm going to explain why this view is completely wrong.

First, there is physical appearance.

On a first glance, Ansem may more closely resemble Xehanort due to his similar fashion sensibilities, but upon a second inspection, his face has its similarities, but also its differences.  His hair and skin color is Xehanort enough, but many of his other facial features are a mix of Xehanort and Terra.  First there are the eyebrows, which have a shape halfway between that of Xehanort's and Terra's, and the same is true about his chin.  As for the nose, it's more prominent and pointy, like Terra's, and he has round Terra ears.

As for his personality and motivations, they still lean most strongly towards that of Xehanort's, but not without Terra's distinctive streak of seeking out more wisdom in the process (not that gaining more wisdom ever seems to do Terra any good; his learning curve is flatter than Larxene's chest).

All in stark contrast to Xemnas.

Xemnas has prominent cheekbones and a straight, flat nose, his chin is very much the same as Xehanort's.

So if he's at least partially Terra's body, why does he look almost exactly like a 30-year-old Xehanort, and also act very much as his own person?

This boils down to what Nobodies are.

Nobodies are more than just reanimated bodies without hearts.  They are actually primarily the outlines, the impacts that hearts leave in this world, inhabiting the bodies left behind.  To understand this, try imagining making a footprint in the sand.  Then for some reason, your body loses your heart to darkness, and it triggers something in the sandy footprint.  The old body falls apart and rebuilds itself from scratch, but without the heart, in the footprint.

The result is that Nobodies won't always look identical to their complete counterparts if their complete counterparts were storing other hearts.  Case in point? Roxas.  Sora stored part of Ventus' heart inside of himself, and so when he became a Heartless, the imprint Ventus' heart made on the multiverse took over the fading body, giving it a new life and personality of its own, resulting in an individual easily distuishable from both Sora and Ventus.

Xemnas is no different.

Terra-Xehanort, or as he's more popularly known, "Terranort", had elements of the hearts of both Xehanort, and Terra.  As a direct result, the Heartless is both Xehanort and Terra, but Xemnas is different.  Xemnas may be Terra's body, but the imprint of his heart animating him is Xehanort's, resulting in an almost wholly Xehanortian appearance, and yet, because he was just a heart outline that filled itself with its own experiences and connections, Xemnas has come to assume much of his own identity apart from those of both Xehanort and Terra.