Of the five Voltron pilots, Shiro is the leader. He was chosen by the Black Lion which is the literal head of Voltron.
Not only that, but Shiro is the most trained of the group. He was a pilot and soldier long before he was a Paladin, and, as we find out, he was a skilled gladiator. Albeit, he was made one against his will.
He was one of the first prisoners taken from our solar system when a Galra ship entered our system looking for one of the lions. They found Shiro, along with two of Pidge's family members, on one of the moons of Pluto, and decided to capture and torture them for information on Voltron. Later, Shiro was put into gladiator matches for the "honor" of Emperor Zarkon. He was undefeated during his time.
Though still fully unclear as to how, Shiro escaped the prison ship and made it back to an academy base to warn other of the incoming invasion. No one believed him though. Except for Keith and the other Paladins, who rescued him.
Shiro is calm most of the time, showing great patience, and a willingness to tolerate nearly anything. He rarely ever gets angry, and when he does, it's usually justified. He's also very methodical, often seeing enemy weaknesses, or realizing what needs to be done to survive or save the day.
If there is one thing that does get to Shiro, it's his spotty memory about his capture. It only comes back in pieces, and the fact that he is literally fused with Galra tech often frightens him. Despite this, he pushes through, and is always there for the team when it needs him. Whether it be in the field of battle, or when they are going through a personal issue.