What does your main character look like?
Is she tall? Does she have dark hair? Is he ugly beyond belief or does he turn heads when he enters a room? Is she a child no older than 10?
In the end, it doesn't truly matter. Your character's physical traits can have an impact on how the reader interprets or reacts to your MC, but all in all it's really what's inside that counts.
I know, I know, that's really lame but hear me out.
I'll try to break down what a great MC has that makes him memorable and someone the reader can cheer on. In short, what does it take to make your MC a hero? Not necessarily in the traditional sense of the word, but in the overall feeling that your character exudes. We all love great villains but in the end most of us want the good guy to triumph. So what are some elements that create someone we can root for?
Your MC has to be a mirror. Not entirely, he or she can be a shard or a sliver, but they must reflect a little of the reader to be compelling. Now don't mistake relatable for likable, those are two different things. There has been some very memorable and interesting MCs that you want to see win the day that haven't been all that nice. Sometimes they're called the anti-hero and can be used in that way to great affect. Having a relatable MC is crucial to capturing a reader's attention. Pour a little pathos into their words and actions and you'll be doing just fine.
Some of the best MCs have the greatest challenges facing them throughout the story, and I don't necessarily mean outside threats or problems that they encounter during the plot. I'm talking about internal conflicts. These usually fall into a sub story or sub text. Does your MC have a chronic disease that she struggles with day in and day out? Does he have a broken past marred by a hideous mistake? Is he damaged in some way that keeps him from what he wants? These are the internal challenges that make a character live and breath.