Dragons Rioting, Vol. 4 (Dragons Rioting, 4);

This looks like a job for Furuhata...I mean, Rintaro Tachibana!
A swimsuit thief strikes! But when Rintaro searches for the culprit, he accidentally accuses a legendary dragon: the Dragon of Severe Waves Ren--who is also the Dragon of Storming Jade Rino's older sister! While conducting her own investigation, Keiko is attacked by a mysterious black beast! Now there's a black shadow enveloping the school...! Oh boy, the culprit this time around is seriously tough. This looks like a job for Furuhata...er, that is, Rintaro Tachibana.