Complex sentence: He and his companion made the first appearance when they found the body of a monster that Saitama killed moments before; while they were isolating the corpse to maintain the civilians away, they were spotted by some passers and took credit for the monster's death.
Split into simpler sentences: He and his companion made the first appearance when they found the body of a monster that Saitama killed moments before. While they were isolating the corpse to maintain the civilians away, they were spotted by some passers and took credit for the monster's death.