Sherra's story goes something like this: After the deaths of Hellmaster Phibrizzo and Demon-dragon King Gaav, Dynast Grausherra comes forward and begins causing problems for humans. Being a mazoku lord, he sends his general to do it for him. Lina and Gourry first meet up with Sherra when she's being chased by two men in black in a village around the city of Bezerdo. They defeat the men in black and 'save' her, but by the next morning she has disappeared from the village. She meets up with them again, and they go to Bezerdo, where they'd heard of sword able to turn people into mazoku. Sherra is caught, but then, much to everyone's surprise, she orders the sword to make someone who was trying to take it into one. After a battle with the new Monster, Sherra reveals herself to be the general of Dynast Grausherra. (Then Lina teases her about the creativity of the name. ^_^ Go figure.)
Later, in the kingdom of Dilss, they hear of a female soldier who has risen quickly in rank to general. Yep, when they get there they find a bunch more battles and plot twists, which eventually leads to the king. When they get him alone he orders Sherra to kill them and leaves. She fights them and eventually dies, with a smile on her face - Lina is confused by this, until she discovers that the king was really Dynast Grausherra himself in disguise, and his General died protecting him.
Sherra's personality, from what I can tell, is one of utmost loyalty, easily willing to sacrifice herself for her master. She lacks a sense of humor and she tends to take things very seriously. Like all Monsters, she looks down on humans and thinks them weak and inferior. Like Dynast, she is very proud.
In looks, Sherra is known mostly for her blue, braided hair. I think in the novels it's said to be black. I have one really good picture of her, here, which about says all that is to say.
Also, she possesses a magical sword called Dulgofa. It's really a mazoku itself, one that Sherra controls. Dulgofa has the power to shoot magical attacks and dispell magic attack spells. However, it's main ability is to turn humans into Monsters. Sherra is a very good sword fighter, just not as good a one as Gourry, which eventually leads to her death.
(Sedalia sent me this transcript of a post asking for Sherra information from somebody that QP answered. I have it up here.)
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As always, If you have anything else that I've left out, please email me if you can. Thank you, and I hope that clears some stuff up for an extremely obscure character!
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Sedalia's Slayers Page - Sherra shrine! YAY! She also sent me a copy of what QP had said in a post about Sherra.
QP/Diana's Slayers Site - Where you can read the full version of what all happened for more details. Also where the actual novel bit came from. ^_^