As revealed in "The Tale of Benthomaar", Benthomaar was left orphaned as a child under mysterious circumstances and was found by King Trimaar and his son Kalmaar in the Endless Sea. Seeing the youngster menaced by giant eels, Trimaar leapt to his defense over the protests of an uncaring Kalmaar. Recognizing the child's plight, Trimaar decided to adopt him, much to Kalmaar's disgust, and gave him the name of Benthomaar. Bentho grew up much loved by Trimaar, but sadly Kalmaar was more interested in his obsession with Wojira than with bonding with his adopted brother.
Benthomaar made various efforts to win over Kalmaar, but was constantly rejected and dismissed. Eventually, however, Kalmaar deemed Bentho, a skilled swimmer, as a perfect pawn in his efforts to find a passable route into the Tartarus Trench and the Temple of Wojira. Feigning regret for his previous unkindness, Kalmaar convinced Benthomaar to help him reach the temple at risk to his own life. He then informed Benthomaar of the truth behind this scheme and his own contempt for his adopted sibling, and Bentho at last saw that Kalmaar would never regard him as a true brother.
Despite this, Benthomaar did his best to treat Kalmaar civilly, and was unwilling to believe the worst about him. In "The Wrath of Kalmaar", Bentho believes Kalmaar when he disclaims responsibility for attacks on surface vessels. Tragically, in "Long Live the King", he arrives in the throne room to find King Trimaar struck down and badly wounded, with Kalmaar calling for vengeance upon the supposed perpetrators: the crew of the Hydro Bounty. However, Trimaar reveals that Kalmaar is actually responsible for the attack, and urges Benthomaar to help the surface-dwellers.
Trimaar expresses his love to Bentho, the adopted son who loved him more than his own biological son, before passing away. In "Escape from Merlopia", Bentho honors his father's wishes by helping the Bounty crew escape the Ripper Sharks. After informing them of Trimaar's fate, he led them down to their ship, deceiving some of the Maaray Guards and helping the Ninja fight their way through the others. Using the Bounty, they managed to escape from Merlopia, and the now homeless Benthomaar was welcomed by his new friends: Nya, Jay, Lloyd, Zane, Pixal, and Maya. Bentho shared his story with these new friends, and joined in their quest to help stop Kalmaar from awakening Wojira.
In "The Storm Amulet", .
In "Riddle of the Sphinx", Bentho reminds his stranded crewmates of the legend that Nyad could communicate with whales, convincing Nya to attempt the same. She proved successful after some difficulty, and in "Master of the Sea" Bentho continues his account, sharing how Nyad sacrificed herself to defeat Wojira millennia ago. The Hydro Bounty crew, pulled by the whales, arrived at the Ninjago City Docks just in time to save the rest of the Ninja, Wu, and Ray from Kalmaar's forces. Bentho refused to side with his adopted brother, and seemingly helped the Ninja secure the stolen Storm Amulet. He was then invited to stay with them at the Monastery of Spinjitzu, unaware that Kalmaar had tricked them with a fake amulet.
In "The Calm Before the Storm", Benthomaar joins Wu, Nya, and Jay at the headquarters of the Ninjago City Police to interrogate Glutinous, Kalmaar's former chief scientist. He exposes Kalmaar's deception and reveals that he has awakened Wojira and is goading her towards Ninjago City. In "Assault on Ninjago City", Bentho joins the Ninja in battling Kalmaar's forces as they invade the flooded city. He tries to convince the Maaray Guards that Kalmaar murdered Trimaar, but Gripe denounces him as a liar and pawn of the surface-dwellers. He and Cole, the latter newly returned from Shintaro, continue the battle while Kai and Zane go to help fight Wojira herself.
In "Nyad", Bentho and Cole rescue Jay from his flooded vehicle, and bring him to a hiding place where they meet the rest of the team. As Jay is suffering from a quantity of water swallowed during his ordeal, Nya questions Benthomaar about Nyad's defeat of Wojira. After reminding Nya that Nyad merged with the sea, he urges her not to take the same action, reminding her that Nyad lost her humanity in the process. However, Nya decides that the price is worth paying to save Jay's life and stop the threat of Kalmaar and Wojira, leaving Bentho and the others no choice but to follow her as she goes to confront the two monsters.
In "The Turn of the Tide", Bentho and the others join in the final battle fighting against Gripe and the Maaray Guards, and after Jay is defeated in battle by Kalmaar, Bentho leaps into the fight against his father's killer. Kalmaar proves too much for Bentho, snapping his spear and demanding that his former would be brother address him as king. Bentho refuses, and as he drops from Wojira throws another weapon to shatter Kalmaar's trident and knock the stolen crown from his head. Bentho then looked on as Nya, surviving thanks to her elemental nature, defeats Wojira and pushes the water out of Ninjago City before departing into the depths of the ocean. He later attends the memorial service held in Nya's honor at the monastery.
In "Farewell the Sea", Cole mentions that Benthomaar spent some time searching for Nya after she merged with the ocean, but eventually gave up and returned to Merlopia to take up his adoptive father's throne.
- He is portrayed by Cole Howard, who also voices Spider-Man in LEGO Marvel Avengers: Climate Conundrum.
- 71755 Temple of the Endless Sea
- 892285 Benthomaar
- Ninjago: Masters of Spinjitzu