Warning: Spoilers ahead for One Punch Man Chapter 135
One Punch Man has finally revealed the Hero Association's No. 1 S-Class hero, Blast. One Punch Man is manga written by ONE and drawn by Yusuke Murata. It has been serialized since June 2012 and has been adapted into an anime. It is a parody series that plays around usual tropes of a superhero story. The main joke of the series is that the protagonist, Saitama, has exceeded the strength of every living being by religiously following his infamous exercise routine -- 100 pushups, 100 sit ups, 100 squats, and 10 km run daily. Because of this, he finishes all his fights with a single punch, thus the title.
When Saitama realizes that there isn't any viable opponent for him, he becomes bored and starts living a seemingly ordinary life in a town outside the Hero Association's radar. He has also lost any interest with the Hero Association since then, but regains interest after learning of a hero's salary. However, he still only falls to C-Class for flunking the written exams. On the other hand, his sidekick and self-acclaimed student, Genos, becomes S-Class. However, Genos is still only the no. 14 among the S-Class heroes. Up until the latest updates, One Punch Man has only mentioned or showed the silhouette of Blast, the no. 1 S-Class hero. The Hero Association relies more on Tornado of Terror, an extremely strong psychic and Fubuki's older sister, in times of crisis.
In Chapter 135 of the manga, One Punch Man finally reveals Blast's appearance in his superhero costume. During the fight with Psychos, Tornado is already on the brink of collapse after literally wringing an entire city. But while she's unconscious, she dreams a memory of being sold by her parents. That's when she first meets Blast, and this is when the series first shows Blast in full detail.
Blast rescues Tornado and gives her a little pep talk. He tells her that those who possess great power does not rely on anybody else to save them. Not only that, he also reminds her that there is, in fact, a person she must protect—her sister.
Even though the series has finally shown Blast, fans are yet to see him in action. After all, he's only been revealed through Tornado's flashback. One Punch Man has also revealed that Blast has already retired from being a hero and is unlikely to join the fight against the Monster Association. However, that doesn't necessarily mean he won't be appearing in future arcs.