She was voiced by Alyson Leigh Rosenfeld.
Rio Kastle was known as Marin in her past life, she was a priestess as well as a princess of the United Lands of the Poseidon Ocean alongside her brother Nash. One fateful day their kingdom was attacked by a ruthless prince known as Vector, who led a army of soldiers to plunder the castle by using Gorgonic Guardians which in turn failed. and then Vector killed one his men and used him as a sacrifice to summon Abyss turning things in his favor, and Marin decided to offer her soul to Abyss and sacrifice herself then plunged herself into the depths of the ocean and summoned Number 94: Crystalzero to purify Abyss which was due to the Barian deity Don Thousand Influencing her and was later revived as one of the Seven Barian Emperors.
At another time, Rio and Reginald visited a theme park where the former offered the latter one of Kastle twins' two rings which they both could wear. Reginald initially refused to wear his ring, causing Rio to throw them on the ground and walk away in anger.
- Barian Battlemorph
- Sensing the presence of other Barians
- Seeing Astral inside or outside the Different Dimension Airship
- Have visions
unbeknownst to her, Bronk has secretly fallen in love with her and they become very good friends since she met him and his friends, but unfortunately when she later returned to her Barian identity as Marin alongside her brother then turned against Yuma & the others which was sad for her cause she shed a tear after defeating Bronk and when he was sent to Barian World.
Despite the two being enemies, they don't have negative qualms towards each other. Dumon felt that if she and Shark were really Marin and Nash he could never hold a serious grudge against them due to their lost memories of their true Barian identities. Meanwhile during her comatose state after the encounter with Abyss, Rio had no reason to oppose him when he successfully attempted to convince the siblings that they were the missing Barians he was looking for, meaning he was trying to help them.
Her and Tori are very good friends, they became friends on her first day back to school they talk on friendly terms. Rio ate lunch with Tori and her friends including her brother Reginald, they look up to each other. She accompanied her and the others to Barian World to rescue Ray and watched the duels that Reginald, Kite and Yuma went through together.
- She owns three Barian Cards called Number 103: Ragnazero and Number C103: Ragnafinity and a Spell card called Rank-Up-Magic the Seventh One.
- She does not share the same personality as her brother.