After Kakua visited the emperor he disappeared and no one knew what became of him. He was the first Japanese to study Zen in China, but since he showed nothing of it, save one note, he is not remembered for having brought Zen into his country.

Kakua visited China and accepted the true teaching. He did not travel while he was there. Meditating constantly, he lived on a
remote part of a mountain. Whenever people found him and asked him to preach he would say a few words and then move to another part of the mountain where he could be found less easily.

The emperor heard about Kakua when he returned to Japan and
asked him to preach Zen for his edification and that of his subjects.

Kakua stood before the emperor in silence. He then produced a flute from the folds of his robe, and blew one short note. Bowing politely, he disappeared.

http://www.ashidakim.com/zenkoans/68onenoteofzen.html