"Sis" Chase is Robert Chase's much younger sister.  She is an unseen character, but Chase talks to her at the end of Better Half.

She is the second child of Rowan Chase and his first wife, Chase's mother.  However, she is much younger than Chase.  It is implied that she was born shortly before Rowan left the family.  As Chase's mother was an alcoholic, much of the responsibility for caring for her fell on Chase's shoulders.

Chase did not begrudge having to take care of her, but when his mother died and he turned 18, he left for the seminary, leaving his sister behind.  It is unknown who took care of her after that point, but it is clear that she resented Robert for leaving.  As a teenager, she took out her anger and, like Chase and her mother, abused alcohol and drugs.  She and Chase only had intermittent contact during this time.  When she reached adulthood, she straightened out somewhat, but remained estranged from the rest of her family.

At the end of Better Half, Chase decides to call her.  She calls him "Robby" and is surprised to hear from him, remarking that she hasn't heard from him in years.  She figures something must be wrong, but Chase tells her he just wanted to talk with her.

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