That No-Good Guild Clerk is Actually an S-Rank Summoner. To Clock Out on Time, I Beat The Bosses in Secret Then Got Kicked Out

Kilt, who lives with his much younger sister, Niina, works as a receptionist at a certain adventurers' guild. On the surface, he is a gentle young man who always carries himself modestly, and because of that, he is constantly looked down upon by those around him.
But in truth, his hidden identity is that of the legendary S-Rank adventurer known as the "Black-Silver Summoner," capable of bringing down any monster in an instant. Hiding his true identity from everyone, he spends his days secretly slaying high-ranking monsters.
Thanks in no small part to Kilt's behind-the-scenes support, the guild had managed to stay afloat, and yet...

"As of today, you're fired."

One day, he is suddenly dismissed by the guild master, who knows absolutely nothing about his true identity――.