Saitama, also known as "Caped Baldy", is a hero ranked 7th in the B-Class of the Hero Association (based on the anime adaptation). He currently resides in Z-City.

Despite passing the Hero Certification Exam alongside the cyborg Genos, who entered the Hero Association as an S-Class hero, Saitama started his hero career as a low-ranking C-Class 388th hero. He is often disregarded due to his low rank, and sometimes even falsely accused of stealing credit from S-Class heroes for his rapid rise in rank. 

However, Saitama is actually the strongest being in the entire One-Punch Man universe. He possesses superhuman strength achieved through relentless training, having broken his limiter and surpassed all human limitations. This power renders him virtually unbeatable. Most monsters are pulverized with a single punch from Saitama, leaving no trace behind. 

His intense training has ironically made him so strong that he has lost all sense of fear or excitement in battles, rendering his life devoid of any real danger.

Despite his unparalleled strength, his rank and popularity don't reflect his abilities. This is partly due to Saitama's unassuming nature and unwillingness to boast about his achievements. His seemingly vacant expression also contributes to people underestimating him. 
Saitama doesn't care much about his rank and simply enjoys being a hero. He shows little interest in his ranking even when it rises, and even remains friends with King, an S-Class hero who has taken credit for Saitama's heroic feats.

Incidentally, Z-City is now a ghost town, and it's all thanks to Saitama. After he wiped out all the monsters in the city, the news of his presence spread among the monster community. Consequently, Z-City became a hotbed for monster activity, leading to the exodus of its human residents. (*This resulted in a deserted city with intact infrastructure, including electricity, gas, and water supplies. According to Fubuki in the original manga, Saitama lives there rent-free.*) Subsequently, monsters visiting Z-City never returned, reinforcing the city's reputation as a monster-infested ghost town.

It's worth noting that upon joining the Hero Association, Saitama was told by Genos that each hero receives a unique codename based on their distinct characteristics. Worried about getting a name like "Caped Baldy," he was ultimately given that very name by the association. This name is a source of much irritation for Saitama, who always introduces himself as "Saitama" and ignores any attempts to call him “Caped Baldy.”

Caped Baldy's Abilities:

• Normal Punch: Saitama's basic punch, delivered with a seemingly effortless single hand. Despite its casual appearance, it's capable of obliterating most monsters with a single blow. Even Orochi, the strongest known monster, was reduced to just a heart after a single Normal Punch from Saitama.
  

  

• Consecutive Normal Punches: A series of Normal Punches delivered in rapid succession, similar to a boxing jab. It is rarely used as most monsters are easily defeated by a single Normal Punch. However, Saitama resorts to this technique against tougher opponents who can withstand a single punch or when enraged by those who destroy his apartment. While typically performed with one hand, Saitama can also deliver "Consecutive Normal Punches" with both hands simultaneously.

  

  

  

• Serious Punch: Saitama's serious punch delivered with a focused stance and powerful fist. It's a stronger version of his Normal Punch. The sheer power of this attack is evident in its ability to instantly kill the Elder Centipede, a creature that even the S-Class hero Blast couldn't fully defeat. The shockwave generated by a Serious Punch can even carve a hole in a mountain and split the clouds in the sky.

  

  

  

• Serious Side Hop: Saitama moves forward in a rapid side-to-side hopping motion with a serious expression. While this might seem like a simple movement, its speed generates powerful shockwaves capable of pulverizing opponents. Sonic, a renowned speedster who had mastered the art of creating ten clones to increase his speed, was effortlessly defeated with a single Serious Side Hop. In fact, Saitama's shockwaves created even more afterimages than Sonic's own ultimate technique.