Mashiba appears in the manga for the first time when, new professional player too, he comes to attend Akira Toya's game, his year mate, at the Shin Shodan Series. Immediately, the character appears to us like very unpleasant.

Only Insei who passed professional with Akira.

Mashiba was a middle level Insei who, much to everyone surprise, passed the professional examination with Akira. Master Shinoda (Insei teacher) himself has been astonished of his success. Nevertheless he has lost his game against Isumi, at the time already first of the class and against Akira Toya, naturally.
He is said to be often lucky, and Amano tells him anyway that his luck take part in his talent.

Boasting and Jealous.

To become pro has given him the large head. He already imagines he's a rival for Akira Toya, but he will not delude oneself for a long time.
Mashiba likes to come to scoff at his ancient mates Insei, as he did at the Young Lions Tournament. He particularly likes to provoke the gentle Isumi who still doesn't answer at his digs. Jealous, Mashiba…
When the others young professional players wear casuals, he comes to this Tournament with a costume to show off.
He appears anyway so disagreeable that Waya doesn't hesitate to hit him.

A second-rate pro without great promise.

The luck he knew at the professional examination doesn't seem to be renewing for Mashiba, and when we can see him at the end of the manga, he stagnates in the lower categories. He predicts himself that Isumi will go beyond him in pro World without delay.
Before that he has been defeated easily as pro by Isumi at the Young Lions Tournament, when Isumi was still Insei.

Isumi: