Haidora in the original Japanese, she is a younger sister of Valkyrie, who she chased her to Earth. She knew that if Valkyrie ran away she would have to take her sister&#039;s place at the wedding which was not something she wanted to do; however she crash landed on Akina&#039;s temple, only to have her powers sealed by Akina making her become a child just like Valkyrie. However unlike Valkyrie her mind is that of her adult form, not a child&#039;s. Much like a kiss can restore Valkyrie, the seal on Hydra can be released at Akina&#039;s will. Hydra has a short temper and is rather tomboyish acting in a rash manner that shows action before thinking things through meaning she is prone to destroy things that get in her way. She can transform into a winged pink and yellow horse and has several other powers. She has taken up kendo while living with Akina, and often pokes fun at Akina&#039;s breast size and her feelings for Kazuto.    When the friendship between Hydra and Akina breaks down, they can combine into a single person, Akidra.

UFO Princess Warukyure, aka UFO Princess Walkyrie is about a princess from outer space who accidentally crashes on earth, where Kazuto desperately tries to maintain the public bath of his grandfather. Due to circumstances, Kazuto receives part of princess Walkyrie's soul which forces her to stay there with him. But that's not the only problem ... because her soul lost strength, the princess transforms both mentally and physically into a little kid! 
(Source: AniDB)

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Valkyrie, Kazuto, Akina, Hydra and friends are back for more when the Key of time suddenly appears along with many other mysterious happenings. All seemed to be linked to a person named `Valkyrie Ghost` who`s only goal is stealing Kazuto for herself. As they team up against this seeming unbeatable foe they dig deeper into the mysterious past of Valkyrie`s home planet, Valhalla. 
(Source: Anime News Network)

The queen of planet Valhalla arrives on Earth with a virtual-world-program, called "bride training boot camp". They built several pairs which have to take a random card with a special situation that might occur if you are married. For instance Kazuto and Valkyrie have to make dinner, Raine and Kazuto's sister have to manage poverty or Akina and Hydra have to prepare a wedding ceremony as bride and groom (so everyone gets the most unlikely and unsuitable situation).
 
(Source: Anime News Network)

A spaceship lands on a bathhouse which belongs to Kazuto's family. Inside is Princess Warukure (Valkyrie) who gives part of her soul to Kazuto to save him from death (caused by her crash). 
Since she only has half of her soul, she turns into her child self. Now Kazuto and company must figure out how to fix the bathhouse and change Warukure back to normal. 
(Source: Cidsa)