Serilda

Serilda, also known as Princess Marina's Warrior Form, is an alternate form taking the shape of a young strong swordsman. It is one of the youngest forms of Marina. This form contains the warrior's spirit.

Background
In ancient times, it was known the young warrior was actually a young boy from a noble family. Due to his resemblance of a young girl, the name "Serilda" was given to him. As said in legends, the young hero had saved his homeland. However, the young boy's fate was unknown, implied to have lost his spirit.

Serilda Reborn
Eventually, he let his spirit rest within Marina, the princess of Sea Side, allowing her to be able to transform in her Warrior Form as Serilda. It was held as secret that Serilda was indeed reborn as Marina now.

Trivia

 * Serilda is a character based on Princess from Magical Pop'n.
 * This form had become more feminine by Marina, due to the fact that she's a girl. But the gender of Marina as Serilda is confirmed to be female.
 * Serilda is somehow similar to the character Sheik from The Legend of Zelda: Ocarina of Time, since Sheik is also an alternate form taking the shape of a strong man, but somehow feminine at the same time. Like Sheik, Serilda is also referred to as "he".
 * Serilda is also similar to Deku Link from Majora's Mask, as the young Deku kid had lost his spirit which rest within a mask the young hero uses during his personal quest.