Professor Tanabe is a physicist, whom we lovingly refer to as "that Geiger-counter guy." Seriously, he's never seen without it; does he sleep with the thing?
Tanabe accompanies Professor Yamane to investigate Odo Island after Godzilla's attack. He reads radioactive fallout throughout the village, and his follow-up research allows Yamane to piece together that Godzilla was awoken by U.S. H-bomb testing in the Pacific.
He's makes a couple other appearances throughout the film. For example, he volunteers at the field hospital after Godzilla's Tokyo attack. He also uses his trusty Geiger counter to find Godzilla's crib on the ocean floor, so Serizawa can go down there and blow the house up, meaning he basically saves the world. Huh, maybe Geiger-Counter Guy could be his superhero name? That would be rad.