I will only talk about the 3 main characters because there are a lot, there also might be spoilers if i talk about all of them.
Eren yeager – Main protagonist of the anime. Only son of Grisha and Kaluga yeager. Adoptive brother of Mikasa Ackerman. He possesses extreme amount of hate towards the Titans after seeing his mother beheaded and devoured by one, he joins and trains in the army with his childhood friends Mikasa and Armin and swears to wipe out the all the titans.
Mikasa Ackerman – Main female protagonist of the series. She is Goddess of awesome and winning. A prodigy soldier, never seems to lose her cool no matter how bleak or hopeless the situation, graduated in the academy with the highest marks. She suffers only from few scratches no matter how dire the situation is while everyone loses their heads, limbs and get devoured. She has strong feelings towards Eren and is very protective of him to the point where she would kill and hold grudges to anyone who would endanger his life.
Armin Arlert – A childhood friend of Eren’s. Wishes to see and explore the world outside the walls. Has a very low opinion of himself and thinks hes useless because of constant bullying when he was a child due to his weak body. He starts to see his worth when he shows of his tactical ability in battle. He would rather die than be a burden to his comrades.