Valentine are a specific type of Backyard life-form created by the Universal Will in order to influence the human world. Emotionless and completely dedicated to the orders of their "Mother", they were created as genetic copies of Aria Hale.
Like other life forms birthed in the Backyard, Valentines are devoid of emotions. They have no values of their own, no concept of right or wrong, and they neither like or dislike anything. It is because of this that they view the orders from the one called "Mother" as absolute commands that are not to be questioned. There is no thought or affection, nor any feelings of doubt or restraint—only machine-like obedience of carrying out a mission. The Valentines view themselves as merely tools to be used to achieve a goal.
Valentines, in general, are unable to place value on anything other than their assigned target. Without any thoughts, feelings or desires of their own, they do not possess the capacity for falsehood, making them naive in their honesty. They are said to be incapable of feeling emotion, but this seems to be only at first as all known Valentines were developed emotions at one point or another.
Despite being genetic copies of Aria Hale, they are not always identical in appearance to her, although they are always female. Each Valentine is programmed differently to suit their purpose, and are given combat abilities to match.
Their number of Valentines currently in existence is unknown. In the epilogue of Guilty Gear 2: Overture, however, That Man tells Raven that millions of Valentines are being born each day. That Man was able to somehow to replicate the method and create a Valentine of his own, although with Justice's (also known as the "Scales of Juno") warped memories as basis for her creation.