Shockwave
We all have to die when our time comes, but if we do our duty we don't got regrets!
Media: Yū Yū Hakusho, especifically the live action ;; The plot takes place a few months after the ending.
Relevant AU info: Keiko is dating Yukina, and Yusuke is dating Kurama. Hiei accidentally overheard Yusuke and Kurama talking about their now half lives, so he took the chance to help them and asked Koenma for the Forlorn Hope to get revenge on all of those who kidnapped Yukina, even if Gonzo was already dead. Hiei convinced the rich men to wish to give Yusuke and Kurama their whole lifetime back in exchange for theirs by making them believe that they would get millions and millions of yen, which wasn't true at all. With them dead, Yusuke and Kurama had their whole lifetime back.
Plotline: After rescuing Keiko and Yukina and returning from the island, Koenma offered Kazuma to work with Yusuke as a Spirit Detective too, an offer that Kazuma accepted without a doubt. A few months later, a really strong thunderstorm started, and even after 4 days wouldn't stop. Koenma found out that a Yokai called Kanna was the one causing the storm, and that was disguising himself as a student in a nearby High School (not the same as Keiko and Yusuke). Initially, Koenma wanted Yusuke to take care of it, if it wasn't for the fact that he sent him away with Kurama and Hiei on a mission, so he asked Kazuma to find the Yokai instead.
Kanna's basic info: Kanna is a Yokai that has the ability to control climate changes, but it only works when his emotions are very strong. For example, if Kanna is really happy, then the weather would be sunny, even hot, for as long as he's that happy. He's been living among humans for a very long time. When people start feeling suspicious about him, he disappears for a while until he finds a new disguise; right now, he is a student in a nearby High School.
Kanna's ability to control the weather: Although it might seem like a chaotic thing to be in control of, Kanna's capacity to feel emotions is quite different from human's. It takes a lot of stimulation to make him feel something at all, so causing a big emotion takes a lot of effort. Moreover, Kanna has an earring that allows him to feel that kind of strong emotions without altering the weather. That's why the thunderstorm concerns Koenma, since there might be something bigger behind whatever is making Kanna feel that kind of sadness.
Kazuma's info: He still hangs out with his group of friends, but spends less time with them since he now has to be available to fight Yokai. His Reiki kept getting stronger, now being able to wield two Reiki swords at once. He's also got better at withstanding pain and getting hurt. The most impressive part is that he convinced Hiei to practice and train with him, doing it daily. None of them have missed a training session, and they've grown closer. This closure is what made Hiei comfort Kazuma when Yukina started dating Keiko.
The Blue Oni: His main power is causing sadness to a group or to a single being. He kidnapped Kanna and stole his earring. Using Kanna's powers and his, he is planning to flood the whole city. Kanna escaped from him, but the oni is still using his power on him, as he couldn't retrieve his earring. Kazuma teamed up with Kanna to stop him. First encounter: Kazuma prevented the oni from capturing Kanna again, but couldn't retrieve the earring. Luckily, Kazuma hurt him enough to stop using his power on Kanna. The thunderstorm stopped for a while. Second encounter: The oni tried to use his power on both Kazuma and Kanna. Even though it worked on Kanna, Kazuma didn't feel sad at all; he's felt worse. Kazuma killed the oni and retrieved the earring. The thunderstorm stopped for good.
Later-on info: Kanna kept his disguise even after meeting Yusuke, Kurama and Hiei. However, Keiko and Yukina knew he was a Yokai. When Kanna accidentally used his powers to stop water from getting them wet after a reckless driver wouldn't slow down even after seeing the puddle, Yusuke and Hiei tried to attack him while accusing him of fooling everyone. Kazuma stood between them and Kanna, and Kurama tried to stop them. When Kanna killed a demonic dog which was about to attack Yusuke and Hiei, they apologized. Kurama and Kanna already knew each other; they met at a library. Kanna knew Kurama was a Yokai, but Kurama didn't know Kanna was.