Siobhan McDougal was born to a powerful Gaelic clan leader, the patriarch of Castle Broen. She was the firstborn of the nobleman's children, followed by her younger brother Bevan. As per the clan's traditions, Siobhan was entitled as both heiress to the castle and clan matriarch.

Siobhan was killed by her brother and was cursed. She was sent to the underworld, only to return with great strength and a wail that could kill anyone who heard it. The villagers named her the Silver Banshee. Then her brother Bevan, used an Irish-Scottish ritual to burn her body and her blood so she couldn't return. The only way she could return was if her portrait is revealed.

As a spirit, Siobhan is shown to have a misandrist and mischievous side. She murdered a man for no apparent reason other than the fact he was a couple with a woman she was possessing. When possessing Chloe, she ignored Oliver, her boyfriend and went after Clark, seemingly for sexual pleasure. However, he was able to resist her, prompting Siobhan to possess Lois, who was Oliver's ex-lover; she attempted to kill him, but was stopped by Clark.

As Clark notes, murdering all these men wouldn't undo her own murder. Siobhan acknowledged this, but explained she only did this out of entertainment.

Siobhan takes on the physical appearance of anyone she is possessing, but gives them a white streak of hair.

When Lois Lane accidentally ripped a portrait at the McDougal Inn, the Silver Banshee was released and took over Bridget Black. Siobhan then looks at Bridget's husband, Mark, in a seductive and sexually yearning way.

Siobhan as Bridget Black is a young woman who looked like she was in her 20’s, long brown wavy hair with a white streak of hair due to the possession, brown eyes, wearing blue jeans, an olive green coat over a blueish-gray button up shirt and white undershirt.

Mark sees the white streak in Bridget’s hair and makes a dirty joke Is that a new look or did I just make a white streak in your hair again?