In the movie The Proffessors son Ken is talking to this college age guy who is also seen with the girl and is seen hallucinating in the night club.
He forms the rock & roll party on Mt. Fuji and when Hedorah attacks is the first to be Sludged to death with toxic mud spit.
I don't think he was Ken's older brother as he referred to the Prof. as 'Professor'. My guess is he was a lab assistant or maybe the girl was Ken's older sister and he was the boyfriend.
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actually, he was Dr. Yano's younger brother. I too was confused who he was exactly til I saw the subtitled version. He calls Dr. Yano "brother" when they get out of the car to survey the damage Godzilla and Hedora caused at the dock area.
shareAccording to the cast lists on Japanese movie websites,
he's actually Mrs. Yano's brother, whose full name was Yukio Mônai.
(IMDb lists his name Keuchi which is possible but Mônai is more common reading.
Since his surname was never mentioned in the movie [if my memory serves], I can't tell which is correct.)
Although the movie itself didn't make it very clear,
it was hinted in the Japanese dialogues in the scene sogoishi wrote about.
After Yukio called Dr. Yano brother, Mrs. Yano called him Yukio.
If Yukio was Dr. Yano's brother, she would have called him Yukio-san.
Also, the way she talked to him was more candid than to her husband.
By contrast, when Dr. Yano talked to Yukio, he used the word kimi for "you".
If Yukio was his own brother, he would have used omae (more intimate/rude version of "you"),
and the way he talked to Yukio was too polite to be his elder brother.