A runaway child who misunderstood freedom. Masumi Seki is one of Punpun's childhood friends and is best friends with Shimizu. He has a rough personality and usually has a cool, indifferent expression even when upset. There is a permanent scar above the tip of his nose that is covered by a bandaid for most of elementary school. His family owns Seki Lunches, which used to do business with the now-abandoned miso factory.

Spoiler

Seki begins smoking cigarettes in elementary school. After he and his friends explore the old miso factory, his dropped cigarette causes an explosion while Shimizu is still inside. This instance traumatizes Seki, causing a deep phobia of fire. He faces his fear of fire when Shimizu again is caught in an exploding building with the Pegasus Ensemble, and Seki breaks inside to rescue him. In the epilogue, he is even shown smoking a cigarette.