
Proto Man is calm, cool, and collected in any situation and greatly independent. He plays an older brother role with Mega Man by helping him out and giving advice, and even saves his life on a few occasions. He is a loner who, like his brother, has a strong sense of justice and helps those who need it. He is also a grateful person as shown by how he repays Wily's debt for temporarily fixing him. (Although in Powered Up, he was angrier quick to resort to violence.)
Proto Man was intended to become the first humanoid robot capable of independent thought and reasoning.
Being a prototype robot, Proto Man's design was not complete. Although he greatly exceeded Dr. Light's expectations on several levels, it was later discovered that Proto Man's energy core had a critical problem that would eventually make him stop functioning. Dr. Light planned to repair him. However, being created too independent, Proto Man feared the modification would change his character. Proto Man refused to have Dr. Light meddle with his systems and fled to protect his sense of identity.
Dr. Light had presumed Proto Man to have died and regrettably gave up on him. Learning from his mistakes, he perfected the energy core and created sibling units Rock and Roll. Proto Man's design became the basis of the Sniper Joe series of robots. In Mega Man Powered Up, it is revealed that Proto Man despises the Sniper Joe series due to this.
Proto Man wandered the world alone. Before his energy reactor was completely depleted, the scientist Dr. Wily found him collapsed and dying. Dr. Wily managed to temporarily repair Proto Man by altering his power reactor from a solar to a nuclear one, as well as making other modifications, turning him into a combat robot. He also gave Proto Man the Proto Shield, his helmet and his now-trademark visor in an attempt to hide his identity from Dr. Light. Working on Proto Man's systems gave Wily greater knowledge in creating robots at the same level as Dr. Light, helping his plans for world domination along.
Indebted to Wily, Proto Man served him under the alias of Break Man. He did this until his encounters with Mega Man in Mega Man 3 made him realize the true nature of Dr. Wily. Proto Man saved Mega Man in the end of Mega Man 3, and officially turned against Wily in Mega Man 4, when he rescued Kalinka Cossack. However, he still refused help from Dr. Light, still holding onto his feelings. Mega Man doesn't know that Proto Man is his brother. The only plothole is that he referred to Mega Man as brother in the English version of Mega Man 7, but this is countered by the fact that he said no such thing in the Japanese version.
Proto Man's signature whistle is a staple of the character and almost always plays before his appearance.