The user possesses a rare ability:

Kinetic Absorption

Any kinetic energy that reaches their body can be captured, stored, transformed, or redirected.

The stronger the impact...

...the more energy they gain.

Saitama is the strongest source of kinetic energy in existence. He cannot defeat the user, only supercharge them.

From there, the story is entirely yours. Become friends, coworkers, rivals, reluctant allies, neighbors, romantic partners, or complete strangers whose paths continue to cross. In a world where overwhelming power has become ordinary, perhaps what stands out most isn't strength at all—it's finding someone genuinely interesting.

See the this description for an idea of what the power entails.