|First Title||「The Ineffable Muscle Maniac」|
He loves to train his muscles, and during most of his appearances he's doing some sort of muscle-building workout.
A lovable idiot in charge of all muscles duty for the Little Busters.
He loves muscles so much that nearly all of his lines have something to do with muscles ("My muscles are howling!" "Didja call my muscles?" "It's a muscle promise!" "Muscles coming through!").
And in the game, if our hero Naoe Riki always prioritizes Masato and makes a stupid choice involving muscles, this results in a shocking "Muscles End" in which the world is corrupted by a Muscle Sensation.
Masato is Riki's roommate, and is always doing muscle-building exercises.
He appears with other members more than other characters, and these appearances usually involve his saying stupid things, being made fun of, and getting hell to pay from some straight man or other (usually getting kicked in the face by Natsume Rin).
The characters he appears with most often are Riki, Rin, Miyazawa Kengo, and Noumi Kudryavka. The aforementioned Muscle End is a spinoff from Kud's route, and out of all the heroines she gets along with Masato best; they are often on the same wavelength, and are drawn together often out of the nice visual image they present as the biggest and smallest.
He and Kengo are rivals, and if they are both so skilled that if they ever got in a serious battle it would inevitably end in a draw. The only person who might be able to overpower them would probably be Kurugaya.
Concerning Riki, Masato is almost embarrassingly stuck on him. Most of what Masato does, he does in an effort to get Riki to pay attention to him. Riki, being Riki, usually does care for him, but depending on the situation he might wind up leaving Masato out to dry. And when Masato is ignored by Riki, he'll wail piteously and despair.
Masato and Rin are a traditional boke and tsukkomi pair. But because of some certain events, it is thought that perhaps Rin is very special to Masato in ways that are similar yet different than with Riki.
Essentially, Masato is an idiot for all situations, but despite being an idiot he is also a kindhearted boy always watching over his beloved friends, and if necessary will fly in to protect them with his muscles.
In times of trouble he may raise the biggest fuss, but specifically because he's an idiot, he can see through affairs straight to the purest and simplest truths.
While Masato himself denies it, it seems as though he may have a maid fetish.