Megumi is a very outwardly stoic, aloof, and calculating individual. He usually has a neutral facial expression that he rarely changes and appears to take every situation seriously. This contrasts with Yuji, Nobara, and Satoru's outgoing and goofy personalities, which often annoy Megumi. He's easily irritated and Yuji has noted that Megumi seems like he's always slightly angry.
Even as a child Megumi was very stern, earnest, and thoughtful, to the point where Satoru questioned him.