Sunstar is a stardroid that appeared in Mega Man V. Dr. Wily created the stardroids based off his design. So what is his background? Well I'm going to give more details about this unique robot master.

Sunstar was created by previous generations, possibly decades before the events of Mega Man 1. He was created as a doomsday device. He was created to punish humans for making dangerous weapons. His creators wanted him to destroy everything and over 75% of the population. His makers goal was to destroy everything and the survivors would build from scratch with no need for dangerous weapons like nukes. However, due to him being a doomsday weapon he was sealed away in a ruin.

Dr. Wily unearthed Sunstar and saw how powerful he was. He was made up of alien material so general weapons could not hurt him. Dr. Wily used Sunstar's unique design and created the stardroids. Robot masters with alien material that it takes special weapons to defeat them. Wily released his new creations upon the world in an attempt to conquer it. His plan nearly worked as Terra the leader of the star droids defeated Mega Man. However, Mega Man got new weapons that could damage these powerful new robots.

Eventually Mega Man defeated Wily's new creations. Then he defeated Wily himself at his space station. Wily as a last resort activated Sunstar and told him to destroy Megaman. Instead Sunstar did serious damage to Wily's flying saucer in a single shot. Wily realized that he could not control the independent Sunstar who did not take orders. Wily retreated.

Sunstar then identified himself to Mega Man as the ultimate dooms day weapon. He also stated his order was to destroy inferior life forms for making dangerous weapons.

Sunstar is a formidable opponent. He has lasers as weapons that can do serious damage in one shot. He can also rapidly fire bombs. He can also turn himself into a buzz saw shaped form and use that as a weapon. He also can fire dangerous bombs that can destroy anything including the floor. He is also very fast. He can also shoot fireballs. His most powerful attack is his Nova attack. This attack is were he causes lasers to reign down on everything then he shoots a large laser beam out. A hit from this can do near fatal damage.

His battle with Mega Man seemed like it would be a draw but Mega Man was able to get the upperhand although Mega Man used a lot of energy. Mega Man offers to help Sunstar which surprises him. He even questions why is Megaman helping him. Mega Man tells him because they are both robots. Megaman tells him he only fights when he has too and that he believes their can be peace in the world. Sunstar tells Mega Man he agrees but their battle has damaged his reactor and its going to explode.

Mega Man is forced to leave without Sunstar. Sunstar explodes and takes out Wily's space machine. Its revealed that Mega Man and Wily both escaped the explosion.

Personality wise- He is a very serious character and is ruthless. This is due to him being created for doomsday. He also does not appear to care about death. This is again due to his purpose and he was built to cause mass casualties. On the other hand, he does demonstrate he has self awareness. This is when he states that he agrees with Mega Man that their might could be peace one day. He seems to have some respect in him when he gives Mega Man a chance to leave before he explodes. This implies he might respect strong opponents.

Another point about his personality is he cannot be controlled. Wily thought he could control him and ended up getting attacked. He shows he is a strong minded independent character not taking orders form anyone.

Here is more facts about him from his CD data

Good Point is he is free spirited

Bad Point he has a hard time waking up.

He likes keeping his mouth clean

Dislikes long hours.

His bad point could be a reference due to him being dormant for several decades before he was unsealed. Disliking long hours could be due to the fact he was built to cause destruction in a short amount of time.