Shinra transfers into Tatsuki's class in volume 2. He's impetuous and intent on finding things to add to the museum, which is why he sets a "price" for whatever explanations he provides when solving mysteries. He usually starts his answers with "Welcome to the Wunderkammer" (Cabinet of Curiosities).
The first chapter very clearly establishes the pattern that the later chapters will follow, with the development of the mystery, Shinra's demanding payment to solve the problem, his use of "contract for payment accepted, Welcome to the Wunderkammer", and the final receipt of some kind of payment, which may or may not be what he'd asked for. In the second chapter, we learn that Tatsuki's father likes to test her aikido skills,