Typically you use “of” when talking about facets. And I’ve not seen it commonly being used when referring to persons. The term is used when talking about inanimate objects or intangible ideas.

Why is a statement like “Decisiveness is an important part of leadership” perfectly natural and correct.

To me “leading skills are also a valued facet of a friend” is incorrect and just doesn’t sound in my ear.

When talking about people, we use things like “trait” and “virtue”.

What is leadership skill?

Does leadership skills are included in a friend’s training?

In both the suggestions I gave you above, you can substitute the preposition “in” for “of”

etc.