Kaibutsu Oujo S01E16 - Princess Succession

Riza, feeling unchallenged with her training, returns to the lake resort, restored by the mermen. Hiro, Hime, and Flandre accompanies her there. While cruising on a lake in a motorboat, she then races against Aron, a famous female assassin of the shark family. Riza's motorboat gets into a wreck, and Aron brings her to shore at night, while the others are highly concerned of her whereabouts. Aron happens to be good friends with Lobo Wildman, which Riza is reminded of his death by Hime. After Poseidon finds Riza and informs the other mermen about it, Riza is challenged to spar with Aron. The next morning, Riza and the others leave the lake resort and return to the mansion. However, at that night, Aron shows up at the mansion, revealing that she was sent to kill Hime. Motivated by her half-brother's death, Riza defeats Aron in battle. In her dying words, Aron asks Riza to avenge Wildman for her as well.