Mako Tsunami doesn't appear too often in the series, but he's known for his willingness to let people eat his fish ("Don't worry my friends, there's plenty more where that came from.") and an odd attachment to the ocean, going as far as marrying it in episode 17. Also in episode 17, he denies losing to Yugi, claiming to have used his giant squid monster to swallow Yugi's Dark Magician, "like the end of Pirates of the Caribbean 2, except it was satisfying!". To Yugi and his friends, he is a "Freaky Fish Guy" as he tried to stop Yugi by lobbing his spear at him, which causes Mako to say "I am not a freaky fish guy!". In his interview in episode 17, his screen caption reads 'Local Fisherman/Freaky Fish Guy'. He later married the ocean.
Mako returned in Episode 28 to choke Weevil Underwood. This led to him explaining to Weevil that nobody should play in the ocean "unless they wish to drown". He also revealed that he and his wife, the ocean, have a mutual understanding which allows him to date other large bodies of water, but the ocean is his one true love. This led to Weevil saying: "Dude, you scored with the ocean? What a freaky fish guy," which provoked Mako into strangling Weevil and threatening to murder his entire family for making such a remark.
In episode 33, it was revealed that he divorced the ocean and was now dating the Domino Aquarium. He blamed Joey for his failed marriage due to the fact that Joey called him a "freaky fish guy" who throws harpoons at people, prompting everyone he met afterward to point out such. After Joey realizes that Mako offers more than what other minor characters from season one have to give, he is also told he is not "a freaky fish guy" but "the freaky fish guy" while giving them a catchphrase, the two come to an understanding, prompting Mako to give his rarest card and locator cards to Joey (without even dueling). He also gets back together with the ocean.
Mako has an odd habit to lob harpoons at anyone. So far he has lobbed a harpoon at three people (Yugi, an audience member at the aquarium and Yugi's Grandpa), as well as a baby dolphin, which he mistook for a kraken.
Mako also made a cameo appearance in Naruto: The Abridged Series as Inari's step-father, allowing Inari to eat his fish just like in Yu-Gi-Oh: The Abridged Series, although he is quick to say that he learned his lesson and won't lob his spear like last time. Originally, Tazuna claimed that Mako was killed, but after Naruto proved that the story was fake (parodying Phoenix Wright), Tazuna said that he ran away because Inari's mother is a horrible wife.