Taiki Yamada (山田大樹 Yamada Taiki, やまだ たいき Yamada Taiki) is the main antagonist of Downtown Nekketsu Monogatari. He attended the same middle school as Kunio and Hasebe. He and Hasebe would later attend Reihou Academy, while Kunio entered Nekketsu High. He used to be the Student Council President of Reihou Academy before Mamoru Tōdō took the position and he currently plays the role of his sidekick.
After his defeat, Yamada transfers to Meian High School.
In River City Ransom (the localization of Downtown Nekketsu Monogatari), Yamada was renamed Slick and is revealed to be an old friend of Alex (Kunio in the original game) named Simon. Slick re-appears in the game's sequel, River City Ransom: Underground.
Yamada also appears in Downtown Nekketsu Kōshinkyoku: Soreyuke Daiundōkai and Bikkuri Nekketsu Shin Kiroku! Harukanaru Kin Medal as a flunkey of Tōdō. He also appears in the localization for the latter game, Crash 'n the Boys: Street Challenge as the Vice Captain of Lincoln High School (replacing Ryūji from the original game) and is named Winston "Skip" Hildegard Jr.. He still acts as an underling of Tōdō (now known as Todd, also playable as the captain of Washington High School and replacing Riki) however.
- Yamada appears to have psychokinetic powers, as evidenced by his Art of Yamada / Slick Trick technique, which picks up a bunch of fallen items/people and flings them at his opponent(s).
- In Downtown Nekketsu Monogatari EX, if Kunio comes to the final showdown with any of the Shitennō fighting at his side, Yamada will instantly kill them if he's allowed to finish his opening speech. If Kunio comes with teammates that aren't the Shitennō, Yamada will complain that this was supposed to be a battle between just them, and summons Ryūji, Ryūichi, and Takashi as reinforcements. Both of these can be avoided by attacking him before he finishes his speech.
- In River City Ransom: Underground, Slick was imprisoned in River City Prison after the events of the original game. He now wears an orange prison jumpsuit and has a new technique in which he turns his arm into a mass of flesh. This is a nod to the character Tetsuo from the manga Akira.