He was distinguished by his knowledge, intellect, sense of humor and conversational skills, and although his style has been criticized as wordy and obtuse, he was one of the more popular Japanese writers in the late Shōwa period.

However, his second work, a short story titled Panniku (Panic, 1957) published in the Shin Nippon Bunkaku, caused a sensation for its unusual concept and style.

Kaikō won the prestigious Akutagawa Prize in 1957 with his Hadaka no ōsama (The Naked King), a story critical of the pressures placed on school children by Japan's educational system.

Kaikō is considered a leftist activist, respected in many Indochinese countries, in particular, for his vocal opposition to Japan's support of the United States' policies in Indochina in the 1960s.

These experiences translated into his novel, Kagayakeru yami (Into a Black Sun, 1968), an account of a Japanese journalist experiencing first-hand the life of the Americans and South Vietnamese troops in Vietnam.