Spy Classroom Short Story Collection, Vol. 2 (light novel) — Takemachi

“Klaus -- s you.”
Before her death, Hearth, the leader of the legendary spy team Inferno, bequeathed a key of unknown purpose to Thea after saving her from the brink of despair. Years later, Thea has finally discovered a box the key can open. Inside, she finds Hearth's last testament -- but it's missing a crucial word. Soon, misunderstanding begets misunderstanding as everyone in Lamplight gets sucked into uncovering the mystery of Hearth's note!