In another episode, Semiramis was defeated in combat by Lord Parsamant, whose custom it was to beat, rob, and otherwise ill-treat his captives. Perceval arrived and saved Semiramis from this fate by defeating Parsamant.
As far as can be seen, any resemblance between Sir Semiramis and the legendary queen of Assyria is purely coincidental. Maybe Chrétien just knew the name and liked it.
Lancelot, or Le Chevalier de la Charrete | Chrétien de Troyes, late 12th century
Fourth Continuation of Chrétien’s Perceval | Gerbert de Montreuil, c. 1230