Karoly D'ulin held membership in the enigmatic, military-focused order of female fighters known as the Mistryl Shadow Guards.
During 0 BBY, a mission involving the transportation of the Hammertong prototype met its end in a stormtrooper ambush, which led to Karoly's presence on Tatooine. She and her friend and fellow warrior, Shada D'ukal, were at Mos Eisley Cantina when Luke Skywalker initially encountered Han Solo.
As a component of their endeavor to secure a freighter for transporting a pilfered Hammertong module, the two Mistryl were acting under the guise of the Tonnika sisters. Karoly adopted the identity of Senni Tonnika, while Shada pretended to be Brea Tonnika. This disguise resulted in their brief detention by Colonel Parq, before Riij Winward, an agent of the Rebel Alliance, secured their release. However, a subsequent disagreement about who would receive the Hammertong information led the Mistryl to steal his freighter for transporting the superlaser component, while he absconded with their astromech, D4, and a technical readout of the weapon.
During the Hammertong assignment, Shada perceived Karoly as enthusiastic but somewhat lacking in experience. However, by 19 ABY, Karoly had evolved into a seasoned combatant. The Eleven Elders of Emberlene dispatched her to "retrieve" Shada on Borcorash, as the Mistryl took steps to sever their connections with Mazzic. Shada managed to escape, and Karoly's pursuit led her to Bastion, where she formed an alliance with Major Grodin Tierce against her former adversary, Captain Zothip of the Cavrilhu Pirates.
Following this, Karoly consented to escort Paloma D'asima to Yaga Minor to discuss a potential alliance between the Mistryl and the Imperial Remnant. Nevertheless, when the machinations of Moff Vilim Disra were brought to light, and the false Grand Admiral Thrawn was unmasked as the actor Flim, Karoly returned to Emberlene alongside her leader.
It is probable that Karoly was related to Team Prime's Manda D'ulin, who was killed in the Hammertong base ambush, and also to Naradan D'ulin, a Mistryl from an earlier time.
Angela Staines played the role of Karoly D'ulin, disguised as Senni Tonnika, in Star Wars: Episode IV A New Hope.