Shadow

Although the translation for サード (Sādo) is accepted to be shadow this is false as proved by the titles of chapters 5 and 15. Chapter 5 is シャドウ (Shadō) or Shadow, however シャドウ and サード are different. Chapter 15 is ザ サード (za sādo) or The Third, so サード means Third. This name is fitting because she is the third incarnation of Sheila (Sheila being the first and Copy the second).