Matsuka is a Mu who has unyielding loyalty to Keith Anyan.
Matsuka only thought that his ability to read mind was unique to him and is ignorant about the Mu race. He was able to avoid detection and pass his adult examinations. In order to reduce the chances of being caught he joins a military fleet in a remote region of space, but his kind heart doesn't make him the best soldier.
When Keith arrives at the Soleid fleet to prepare for his mission Matsuka watches him closely because he fears the cold and machine like heath of Keith. Matsuka is assigned to take care of Keith while he is preparing for the mission. While alone with Keith he notices him asking questions in his mind but refrains from answering so he wont give away that he is able to read minds. Matsuka slips up and replies to something Keith thinks and before Keith can take action he is forced back by Matsuka's powers. When he lets up Keith takes action and detains him, Jonah is now fearing for his life. Keith asks him if he is a Mu spy and why he is in the military, Matsuka replies he knows nothing about the Mu and has been hiding from Mother. Keith believes him and allows Matsuka to live after he assumes he is of no threat, but tells him to never use his Mu powers on him again.
Keith chooses for Matsuka to come along to the mission to Silversteen seven. While on the way he acts as Keith's aide. When Keith decides to go down to the planet alone and looses contact with the ship the inexperienced crew panics and decides to leave Keith behind, but Matsuka who has become loyal to Keith for sparing his life says they should wait and send out a search party; a fellow member of the crew knocks Matsuka out from the behind.
When Matsuka awakens he sees that they are back with the main fleet and asks his commanding officer for a ship so he can go back to save Keith, his request is granted and is given a small ship. Matsuka makes his way back to Silversteen Seven and the traps set by the Mu don't work on him due to himself being one of them. He calls out to Keith with the ship's radio but Keith doesn't answer he starts calling out to Keith's mind, going against his orders of never using Mu powers on Keith again. This gets the attention of the Mu on Naska and they ask if he is an ally but they are confused when he is actively helping Keith, enemy of the Mu, and going against his own people. This agitates and implants doubts about Keith in his mind but he continues looking for Keith. Keith finally answers and Matsuka helps Keith escape. While on the way back to the main flee he asks Keith if the people on that planet were the same as him or were they just monsters trying to deceive him, he is told they were monsters just like him. This made Matsuka angry and he starts to choke Keith but Keith reminds Matsuka that he spared his life and Matsuka stops.