I know that she didn't live on the streets but in a boarding school. I see Urchin Background as representing her training. Lying and undercover work.
The Head Mistress took the Not One Child Left Behind craze too much to heart, refused to see her for the meathead she is and did not channel her into the Cult Enforcer (i.e. Dark Justiciar) Course ...
Yeah, Shadowheart does show Baghtru levels of Chaotic Stupid at times. She makes Lae-zel look subtle by comparison ...
By that logic, Karlach should also have Urchin as a background because she used to be an orphaned street urchin.
Truth be told many of the companions should have had their own custom origins. Shadowheart as the Daughter of Darkness with proficiency in Insight and Deception, Karlach as the Slave of the Hells with Proficiency in Athletics and either Insight or Perception and Wyll should have a background known as the Fraud of the Frontiers with proficiency in Survival and Persuasion.
