Douglass is the book's narrator, and it's a book about him. So in a way, it's hard to really describe him as only a character.
His personality is where everything starts. Douglass is a titanic person, a real giant of a man, who persevered through some of the worst kinds of personal circumstances and never let his troubles and adversities take away his humanity. He doesn't just want to get by in this world; he wants to be great.
When the book starts, Douglass is still a child, and he's like most children: sort of oblivious, a little careless, and often getting into trouble. But, like most slaves, he also has no choice but to mature very quickly.
By the time he's a young teenager, in fact, he's dealing with some very adult problems. He's facing adversity in a way we come to expect from him as we get to know him better: he's always clever and resourceful in the face of danger, but forthright and unbending when it comes to questions of morality. Most young slaves learn to give in to the pressures of a hard life, learn to save themselves by going with the flow. But Douglass refuses to bend, to compromise his principles, or to ignore what he knows in his heart is right.
Check it out:
I prefer to be true to myself, even at the hazard of incurring the ridicule of others, rather than to be false, and incur my own abhorrence. From my earliest recollection, I date the entertainment of a deep conviction that slavery would not always be able to hold me within its foul embrace; and in the darkest hours of my career in slavery, this living word of faith and spirit of hope departed not from me, but remained like ministering angels to cheer me through the gloom. This good spirit was from God, and to him I offer thanksgiving and praise. (5.12)
He's probably lucky that his adherence to the truth and his stubbornness didn't put him in an early grave, but then maybe he's also lucky that he was so stubborn. Without that persistent courage, he never would have found his way north to freedom. But finding this balance isn't something that comes easily for him. The older he gets, the more Douglass gets singled out as a stubborn slave, so unyielding and determined in what he feels is right that he's seen as a threat to the whole system. From a very young age, he sees slavery as immoral, and he's not afraid to say so.
Just look at how he describes not only the effects of slavery on slaves themselves, but also on slaveholders:
Slavery proved as injurious to her as it did to me. When I went there, she was a pious, warm, and tender-hearted woman. There was no sorrow or suffering for which she had not a tear. She had bread for the hungry, clothes for the naked, and comfort for every mourner that came within her reach. Slavery soon proved its ability to divest her of these heavenly qualities. Under its influence, the tender heart became stone, and the lamblike disposition gave way to one of tiger-like fierceness. (7.2)
Slavery, as Douglass (quite clearly) sees it, is something that corrupts everything around it.
On the other hand, Douglass never has any desire to be perceived as a martyr. It's important to remember that he only puts his life on the line when he has nothing to lose. Douglass wants to live a full, normal live, and, though he is a crusader and an activist, he isn't a saint. He just wants what white Americans take for granted: life, liberty, and the pursuit of happiness.
He has a few friends, but he also understands that he has to leave his friends behind to become free. It is hard, but he does it. He always puts morality ahead of any other consideration, and his struggle to be free (and to free others) always takes precedence over everything else. As he put it in his later years, "I would unite with anybody to do right and with nobody to do wrong."