Testing Interventions for Borderline Personality Disorder.

INCLUSION CRITERIA 1. Participant with borderline personality disorder (BPD) meets full diagnostic criteria for BPD (i.e., five or more BPD diagnostic criteria per the Diagnostic and Statistical Manual of Mental Disorders-5-TR) 2. Both members are at least 19-years-old 3. Both members consent to study participation 4. Both members consent to a member of the research team contacting one of their provided emergency contacts if there are imminent safety concerns. 5. Both members consent to having assessment interviews and treatment sessions audio- and video-recorded 6. Both members members reside in Ontario (with no plan to leave the province during the course of the study) 7. Both members members are fluent in English 8. Both members are willing to receive emails about the study 9. Both members have regular internet access from a private location for completion of study appointments EXCLUSION: 1. Severe intimate partner violence in their relationship in the past year (Endorsement of severe intimate partner violence items on Conflict Tactics Scale-2) 2. DSM-5 criteria A and B of schizophrenia, not better accounted for by BPD 3. Hospitalization in the past year for mania, or mania in the past three months 4. A substance/alcohol use disorder that is likely to require medical intervention (e.g., detoxification) to reduce use 5. A medical condition that is likely to require hospitalization within the next year 6. Scaled score below 70 on the Test of Premorbid Functioning, suggesting impaired intelligence and/or probable traumatic brain injury 7. Either participant in the dyad is not able to show proof of identification upon request at any point throughout the study or is not able to provide an accurate phone number or address upon request. 8. The individual with BPD is currently receiving an empirically-supported BPD treatment and is unwilling to pause the therapy for the duration of the active treatment phase