Siobhan Mournwyn Amcathra was born into one of the Waterdhavian noble houses. Her mother, Sarabeth Amcathra, is the sister to the current patriarch, Arrivan Amcathra. Her father was an adventurer her mother met in Baldur's Gate, Theargood Mournwyn, who is said to have some lycanthropy in his ancestry. Through her noble blood, she shares common lineage with Mourngrym Amcathra, former lord of Shadowdale. She grew up listening fondly to the stories of his trials and triumphs and resolved to be no burden on his great name. As a young child she was betrothed to a member of House Thann, Culben. However as Culben was a cruel boy and a growing chauvinist, Siobhan made sure to torment the boy each and everytime they crossed paths until he declared he would never married the Amcathra she-devil. Siobhan was thrilled but her family seeing her no longer useful as a lady of the house, sent her off to the forge and stable with her male cousins.

She took well to the lessons of the forge, knew how to spot the flaws of a blade, and her uncles even took the time to teach her some swordsmanship. But as she grew past her gawky teenage years, her noble blood took hold and her beauty became known to the suitors of Waterdeep, even Culben had to cede this fact despite his hatred of his childhood tormentor. Growing tired of them seeking her hand, Siobhan first joined the temple of Tymora, hoping some great fortune would come relieve her of the burden of being a weapon of familial politics. However, as she spurned more and more suitors even in her service to the temple, eventually one of the young men snapped, unable to cope with someone with his stature being denied. He charged at her with a dagger but much to his surprise, Siobhan easily disarmed him and drew a long cut across his back. Shocked at her own reflexive violence, Siobhan quickly covered the wound before the man's retainers could see. A shiver of light glimmered in her mind and her eyes as the goddess of luck granted her a gift. The wound healed leaving no scar under Siobhan's hand. Sensing Tymora's support for her plight, and reassured by the power vested in her faith in the goddess of fortune, Siobhan declared then and there she would marry no man who could be bested so easily in combat.

Siobhan stands about 5'8", has straight blonde hair that reaches down to her shoulders and brilliant blue eyes. Around town she dresses in loose dresses of fine silk, preferring white and blues. When dueling she dons plate armor with oversized golden colored pauldrons and a red painted cuirass. On her left arm she wears a heavy oblong shield she calls Luck, and her right arm is a jabbing, whirlwind of slashing steel she calls Fortune. She had a hand in forging both items and is sentimentally attached to them.

Siobhan is a carefree and jovial individual who grew up a tomboy. She loves fortune tellers and gambling. She is not afraid to get into the thick of battles and despite her noble heritage, does not discriminate against those in lesser standing. In combat she uses biting insults (bolstered by Tymora's divine aid) to force her opponents to face her. It is rumored that when she becomes injured, her eyes develop a dark shadow and her canines take on a savage quality, with several dark lines becoming apparent high on her cheek that resemble large whiskers.