Masamune is very emotionless and expressionless. Having to grow up with a strict mother he has a powerful loyalty to those he becomes affiliated with and basically does nothing until a superior or a leader gives him an order. His behavior and mannerisms resembles that of a robot, being obediant, often silent, observative, analystic, honest and cannot even create a false expression of emotion. Masamune never uses his techniques on opponents he finds weak or helpless, which is why his potential is rarely seen and even allowing Kouta to win in the preliminaries a second after their match started.
He is also quite honorable and battles only those he knows is strong or skilled enough to match him in combat. This was due to his rough training childhood resulting him in becoming the academy's number one rookie shinobi along with his childhood friend Usagiko. Some time after being assigned to Team 9, Masamune begins to develop a deep sense loyalty and care for his team to the point where he makes a promise to Kaede to protect Naruto with his life during the Sasuke Retrieval Arc, battling to the brink of death against Yokai in order for Naruto, Kiba and Shikamaru to continue the mission.
Masamune values the love and nuture of his family and friendship he developed with the Konoha 17. He develops the use of emotions in Shippuden in order to understand his students and the world.