He looked to be in his 40's, he was short yet well built, he had black hair and wore glasses.
He is seen sitting with his wife and daughter at a table watching the television when a news report flashes on, his son Masaru comes down but his wife shushes him. The report goes on to show Ryo Asuka who plays a video he took of Akira Fudo's first transformation into Devilman. When Masaru laughs at the report thinking it a joke, paranoia gets the better of Yoshio and the others think him to be a demon in disguise. This pisses Masaru off and he leaves to his friends Takeshi's house.
After Masaru returns home he is attacked by Mina, she runs upstairs but Yoshio comes out whirling a baseball bat at his son, he misses several times but manages to corner him, however before striking Masaru's Grandfather stops him. Immediately thinking him to be a demon he attempts to hit him, however the old man (who was actually a Devilman) grabs him by the head and throws him into a fireplace breaking his neck and killing him. He would have been bought back to life after Masaru's Grandfather reverted time back to how it was several hours ago.