Phantom Thief Hello Happy, referred to mostly as "Phantom Thief-san", is a self-named phantom thief, who acts as the villain of the "The Phantom Thief of HaroHapi and the Luxury Cruise Ship" event story in BanG Dream! Girls Band Party. He kidnaps Matsubara Kanon, and makes the other members of Hello, Happy World play a number of games in order to get him to return her to them.

"His" true identity is fellow band member Seta Kaoru, a fact which Misaki manages to see determine fairly swiftly, due to the timing of Kaoru's disappearance, along with the Phantom Thief's mannerisms. However, Kokoro, Hagumi, and Kanon remained oblivious to this fact, and never found out. Kanon did however claim that she never felt scared in the Phantom Thief's presence, feeling almost as though "he" was some sort of friend. Kaoru's motivations for her actions were to experience the thrill and fun of the game, and being chased after. In actual fact however, the entire thing was staged by mystery women in black suits, for reasons never revealed, who asked Kaoru to act as the Phantom Thief.

Following the final game, in which Kokoro correctly guessed the Phantom Thief's motivations, "he" returned Kanon. Later on, Kaoru rejoined everyone, claiming to have in been with them the entire time, passing everyone else not noticing her as them being so determined to catch the thief. Misaki was the only one who saw through everything, confronting Kaoru one on one about it. However the situation was all's well that ends well, as the others had had an enjoyable time, and Kaoru was pleased that she got to put smiles on her friends faces through her role as the Phantom Thief.