Who is Kostia from The Case of Comrade Tulayev and what is their importance?

Kostia is a young man who remains optimistic about life, even though he lives in poverty in Moscow. His neighbor Romachkin gives him a gun, and Kostia carries it around with him for days. Finally, one February night, he uses the revolver to assassinate Comrade Tulayev in a random moment when he sees the man on the street. Kostia has a childish personality. He is tall and has a high forehead, unruly hair, and sea-green eyes.