Azumi is Ruka's father.  He works at Enokura Aquarium and lives apart from his family.

One summer vacation, Ruka meets two boys, "Umi" and "Sora," whose upbringing contains strange and wonderful secrets. Drawn to their beautiful swimming, almost more like flying, Ruka and the adults who know them are intertwined in a complex mesh... 
Meanwhile, an unexplained anomaly is occurring all over the world: fish are disappearing. Thus begins a marine adventure of boys and girls to captivate all the senses!

Marc Thompson is an American voice actor who has worked for 4Kids Entertainment, NYAV Post and DuArt Film and Video. Some of his major roles include Kevin Thompson, Anthony DeMartino, Timothy O'Neill in Daria. He is notable for voicing the titular character in Robocar Poli. He voices Casey Jones in the 2003 version of the Teenage Mutant Ninja Turtles cartoons and video games. He is the narrator of numerous Star Wars audio books including The Thrawn Trilogy, Hand of Thrawn, Legacy of the Force and Fate of the Jedi series. He earned his BFA from New York University's Tisch School of the Arts in 1997. He is a Christian and a father of two.

(Source: Wikipedia)

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When Ruka was younger, she saw a ghost in the water at the aquarium where her dad works. Now she feels drawn toward the aquarium and the two mysterious boys she meets there, Umi and Sora. They were raised by dugongs and hear the same strange calls from the sea as she does.
 
Ruka's dad and the other adults who work at the aquarium are only distantly aware of what the children are experiencing as they get caught up in the mystery of the worldwide disappearance of the oceans' fish.
 
(Source: Viz Media)

Masaaki Azumi