He is Luke Skywalker. He is Galahad. He is Yojimbo. He is Superman.
He is a man. In principle, he could be a woman, but this never seems to actually happen.
His virtue is his greatest asset. The Villain may be smarter, wealthier, more powerful, but he will always defeat the Villain because of his innate goodness.
He is spiritual. He is also a warrior.
He is not a sexual being. He values love (and may have a One True Love), but does not put any value on carnality. His ideal love is unrequited. If he does have sex, it is only so that he may have a son to take his place.
He is on a Quest. He may not Put Down the Sword until the Quest is complete.
Once he Puts Down the Sword, he may rest, but there is always the possibility that he may need to Pick Up the Sword again.
He will avenge his comrades but he will not take vengeance. He gains no joy in defeating the Villain. It is simply a moral necessity.
He is a protector of the weak. He may suffer insults but he will never stand for allowing those who are defenseless to be bullied or terrorized. Protecting the weak is the only thing that can detour him from his Quest… but only for a time.
The Quest is his First Love. He may also have a One True Love, but she can not prevent him from Taking up the Sword and Going on a Quest.
He is holy, but he will deny this.
He can be betrayed but never corrupted. Those who become corrupted were only ever False Paladins. This is not a circular observation. The virtues of a True Paladin are innate and incorruptible.
A Paladin can become Lost. He may become confused by evil and lose his way. If this happens, he will need to take a side-quest to redeem himself. Only then can he continue his Quest. Never the less, he remains incorrupt.
His moral universe is stark and simple. He believes in Good and Evil and he knows the difference between them.
Honor is the highest virtue. Without honor, there is no virtue.
He will not take advantage of his enemy’s weaknesses. He will meet his foes only in honorable combat. The notion of sneaking around and stabbing his enemies while they are off-guard is repugnant to him.
He is a soldier and he may fight in battles and in defense of his Liege but, ultimately, he Fights Alone.
It is likely that he has a Mentor. In some rare cases, he will have more than one.
His primary source of education is the Spiritual Quest. It is far more important that he learns how to be virtuous than it is that he learns to be lettered. He values wisdom above knowledge.
He knows God to exist but he does not proselytize. He leads by example.