Tasmia Mallor is from her homeworld which is Talok VIII. She has the power to project darkness. Like all Talokians shee has blue skin and pointed ears. She and her cousin Grev received their powers from their ancestors, whose spirits reside in a cave on Talok VIII . As her ancestors before her from the past thousand years, Tasmia is a hereditary shadow champion of Talok VIII. Her 20th century ancestors, were also shadow champions and members of the interstellar police force L.E.G.I.O.N.

Her cousin Grev, who also wielded the shadow power thanks to his bloodline, went on to join the Legion Academy as Shadow Kid though aware that he could never join the team whilst she was an active member due to the rules regarding members having the same powers , he hoped to be better prepared to protect Talok in Tasmia's absence.

Shadow Lass joined the Legion after her homeworld was invaded by the Fatal Five. She helped the Legion to defeat them. After joining, she became romantically involved with Lar Gand and eventually married him after he was mortally wounded fighting the Time Trapper as part of a major conspiracy to avenge Superboy's death at the Trapper's hands. Their marriage was rocky for many years despite their strong feelings for each other, and Tasmia went into mourning when Gand died.

The mourning was temporarily, as he returned to her, inhabited briefly by the Time Trapper's essence, until a cataclysmic battle erased the Trapper from existence and this completely rewrote reality causing to lose her husband again.