Yuuko is one of Valia's daughters along with her younger sister Valna. Although born in Vanity, she was sent to the human world for unknown reasons (maybe the eternal conflict between the dream world and the dark world) and then adopted by a woman from there when she was a baby. After many years she became a normal highschool student with a pacific life, but that changed when strange creatures attacked her hometown and her. She used the Valis sword that appeared in front of her to protect, she met her true mother (not knowing It yet) afterwards only to know that she is destinated to protect the three worlds from any menace. Thus starting her double life as the Valis warrior as she realized the objetives of Rogles. She had to fight many fierce opponents and watch many people die in front of her eyes, such as her friend Reiko. In the process she discovers her true identity as Valia's daughter and accepts Valna as her sister.
After many time fighting, Yuuko decided to save the Valis sword in Valhalla, outside of the reaches of evil hands, becoming therefore the Valis godess.
She became aware then that a new evil dominator named Galgear was putting the worlds in jeopardy once again and tested a new warrior, Lena Brande, to give her the Valis sword. Lena completed successfully the test and became the new Valis warrior. After this, nothing about Yuuko's fate is known as she still seems to live in Valhalla.
Yuuko is a normal teenage girl. When the series started, she was potrayed as a lazy, shy and fearful girl, although, when needed, she is brave and full of determination. She is also kind and worries about her beloved people (such as Reiko or Valia). She seems to like little animals (such as puppies), toasted bread and fashion (as seen when she puts on any of her suits).
When becoming the Valis godess, Yuuko becomes more reflective and calm, but also becomes highly demanding as seen when testing Lena.
Yuuko is very athletic, she has an innate hability to jump highly with and without the Valis power.
As the Valis warrior, Yuuko can use her sword to attack at range or with different types of offensive shots. She can also use the Fantasm Jewels to cast many spells and is invulnerable to many attacks that would be deadly for normal humans. Within this form she can jump even higher and slide under narrow passages. In Valis SD she can shot with her sword upwards and do a double jump.
As the Valis godess, she can levitate (as Valia did), throw sparks and make copies/ illusions of herself in many forms.
As the main protagonist, Yuuko appeared in all the Valis games and compilations.
During the main series first three games, she changes progressively her normal teenager life to that of the Valis warrior, by fighting many dark emperors and their armys protecting Vecanti and the human world from their ambitions, afterwardly helped by Valna and Cham.
In Valis IV she leaves her protagonist role to Lena, becoming her mentor and the Valis godess.
In Valis SD her role of the main series remains the same, although she is potrayed as a sillier, forgetful and naive girl in a commical way.
Yuuko makes a cameo appearance in another game of Telenet called Dekoboko Densetsu along with Megas, a racing game for the PC-engine. In the opening scene, she appears in a screen in the lower-right part of the control board.
Yuuko is also one of the playable characters from the crossover board game of Telenet named Nariagari Trendy, where she is followed by many other characters from other game series of Telenet.
Yuuko also appears in the two mangas of Valis following a similar role. In the ZOL's manga she seems to be more frightened by the monsters that chase her, but as in the game, she becomes brave within the time and fights them.
- Yuuko is well known for her variety of clothes in many of the series games.
- Yuuko's name means literally "Gentle child/girl", deriavated from the kanjis Yu (優), gentleness; and ko (子), child/girl. This name would fit with her personality as she is a kind girl.
- Yuuko is the only character in the series to use more than one type of weapon.
- Yuuko is the only character from the series whose name starts with 'Y'.
- Yuuko's name was changed to Syd in the localized Syd of Valis.
- Yuuko's surname (Asou) is localized as Ahso in Super Valis IV.
- Yuuko's hair seems to be colored as dark purple in some artworks.
- The only game where Yuuko isn't playable is Valis IV, although via an input code, Lena's suite and hair colour will change to that of Yuuko's one in the SNES version.
- Yuuko's appearance and personality seems to have been inspired by that one of her seiyuu in that momment (Sumi Shimamoto).