Tareo is one of the weakest One Punch Man characters that fans have been introduced to. At a surface level, it might appear that Tareo doesn't play an important role in the series. However, that certainly isn't the case.
This individual profoundly impacted one of the most important characters in the Monster Association arc - Garou. This character was introduced as a major antagonist. However, the plot, as well as the character's progression, takes an odd turn, all thanks to Tareo.
Disclaimer: This article contains massive spoilers from the One Punch Man manga chapters.
One Punch Man: Understanding how one of the weakest characters saved the day in the most unexpected ways

Tareo's character was introduced in the anime series' second season, in which he befriended Garou. He mistook the Hero Hunter for someone who was training to become a hero. However, the martial arts expert had other plans.
Garou wanted to become the symbol of absolute evil because of his perception of heroes. Since he was bullied as a child, he wanted to become the source of fear - something that everyone could unite against in order to stop the unnecessary struggles like bullying that the weaker members of society had to endure.

In chapter 165 of the manga series, he took on Blast and even managed to kill Genos. Garou had joined the dark side and took on a path of abject darkness. This is when he saw Tareo, and he had a brief moment of hesitation. He realized that he could never see Tareo again. However, he told himself that it was something he needed to do.
While he didn't say it outright in the manga series, Tareo certainly played an important role in Garou's shift in behavior. Tareo was also bullied, and Garou could see a bit of himself in that young boy. The child played an important role in the Hero Hunter's character development. During the fight, Garou was recreating the effects of nuclear fission, and the radiation killed many people.

Unknowingly, he also ended up killing Tareo, which he realized during the fight. While Saitama subdued Garou, the damage dealt was irreparable. Genos and Tareo, among plenty of others, were dead. However, it was Tareo that Garou wanted to save first once he became the symbol of absolute evil. These things profoundly impacted Garou, making him teach Saitama to travel back in time.
If this hadn't been done, Genos would have stayed dead, and the Hero Association would have suffered the loss of plenty of talented heroes. Tareo was why Saitama could travel back in time and defeat Garou before he could cause any further damage.
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