Maybe it's the most important trait in a leader, athlete, friend—the list goes on and on—because it's probably the hardest to achieve. Consistency, no matter the subject, is active. Participative. Engaged, invested, ride or die. Okay, maybe not necessarily always the latter, but you get the point.
I can't speak on this without the disclaimer that I am often inconsistent in striving for consistency. I don't think it's a quality that is inherently instinctual—I'm still learning what it looks like in the places it's needed in my life. In short, that's everywhere.
But to offer a less convoluted statement about my own human nature, I'm never not wanting to be more consistent; it's just something that requires remembering at all times, something I am quite guilty of forgetting.
So perhaps it's hypocritical to critique the areas of inconsistency in others' lives in addition to my own. The unfortunate truth is that these things are much easier to observe objectively. The next, self-improving step is attempting to apply the things you wish to surround yourself with to your own behavior.
I have had people come and go in my life, and I am almost certain that the defining factor in their staying (or not) has a little something to do with their well-established disposition and habits or lack thereof. There have been those that have surprised me with a plethora of what are believed to be the best personality traits in a person, but these same people have lacked consistency within these areas. For instance, it's pretty hard to be considered loyal if you waver in doing so. If anything, the inability to maintain this is more harmful to a relationship.
Consistency is so codependent on so many other characteristics that solidify getting you that job, staying on top of your schoolwork, maintaining friendships.
I never realized how vital it is in every setting imaginable until I encountered its antithesis. Whether it be your words or actions, behaving in a constant manner will help you build a character that not only others, but you too can rely on.