Eren is Shingeki no Kyojin's protagonist. His adoptive sister, Mikasa, notes on numerous occasions that he acts on impulse without thinking things through, and she often pulls/carries/throws him when he starts fighting with others to protect him from himself. Along with Mikasa, he tends to spend his free time with their mutual friend, Armin. His father is a doctor (whom is well respected by Hannes and potentially many more) whilst his mother is a housewife.
Mikasa is Eren's adoptive sister and mutual childhood friend of Armin. Mikasa shares an unbreakable bond with Eren following events in their past, and rushes to protect him whenever he is in danger. Mikasa possesses impressive fighting skill and strength even as a child. As a teen, Mikasa is considered a genius among all the army trainees and is incredibly skilled at combat and agility. She constantly worries for Eren's well-being and lives to protect him.
Eren's best friend, he is physically weak yet emotionally strong. Even though lacking in actual combat skills, Armin is a genius in theoretical courses and can make plans even under extreme pressure. Even so, Armin keeps on thinking that he is weak. He himself does not notice the things he does for Eren and Mikasa, and believes himself to be completely incompetent and hates himself for having to rely on Eren and Mikasa.
Titans (巨人 Kyojin?, lit. "Giant(s)") are a race of giant, man-eating humanoids that serve as the catalyst for the events in the series. The people inside the Walls were taught that they first appeared 107 years prior to year 850 and rapidly exterminated humanity to the point of near-extinction. In reality, they are transformed humans known as Subjects of Ymir and have existed for nearly 2,000 years.