This is the combined form of Akina and chibi Hydra. When Akina began experimenting with some new incantations to fight against Valkyrie Ghost, she and Hydra are fused together during an argument. The end result is a white-haired shrine maiden with the powers of a Valhallan princess with mystic seals, who possess both the powers of Akina and Hydra, and has the two of them sharing the same body. Their combined abilities are magnified, enough so that could even defeat Valkyrie Ghost, but there are two drawbacks. In order to combine, Hydra and Akina must be in a state of disagreement, then touch hands. Normally, this is achieved whenever one of them does something to anger the other, usually Akina rattles Hydra by destroying some of Hydra's personal things. They once accidentally fuse during the 3rd season because of an argument. Secondly, if the two are in agreement about anything (even something like not being able to agree on anything), they automatically separate. This condition makes it very difficult for them to cooperate during combat. That said, Akidra is a very effective combatant, managing to defeat Valkyrie Ghost on two occasions (whom was more than a match for Akina and Hydra when they first fought her).    wikipedia

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.<br><br>
(Source: Anime News Network)

The third season doesn`t have a particular story behind everything (like "Valkyrie Ghost" in the second season). It just tells 6 stories about the life of Kazuto &amp; Co. with some additions: Raine and Chorus get a house on earth, the last three Valhalla princesses each get an episode, and in the last episode Kazuto and Valkyrie, who in the same episode managed to stay adult, &gt;try&lt; to marry again. <br><br>
(Source: Anime News Network)