Relationship dysfunction—marked by frequent conflicts—is one of the hallmark features of borderline personality disorder (BPD). However, the BPD couple as a dyad and partner-related features have rarely been taken into account. The aim of the present study was to investigate hormonal, personality, and relationship relevant factors, such as relationship satisfaction, attachment, and trauma in both partners within a dyad where one partner is diagnosed with BPD. The total sample consisted of 26 heterosexual couples. All studies were conducted at 2 p.m. Primary outcomes: Neo-Five-Factor-Inventory, Childhood Trauma Questionnaire, Experiences in Close Relationships Scale. Secondary outcomes: Problem List, Partnership Questionnaire, Questionnaire for Assessing Dyadic Coping. Upon questionnaire completion, one saliva sample was taken via passive drool to assess baseline cortisol and testosterone levels. Results showed that females with BPD have higher scores on childhood maltreatment, dysfunctional attachment styles, and neuroticism than mentally healthy females. Furthermore, they have more relationship-related problems and are less satisfied in their romantic relationship. Male partners of women with BPD showed lower testosterone levels, higher levels of childhood maltreatment, dysfunctional attachment styles, neuroticism, and openness compared with the healthy control partners. Furthermore, childhood trauma, neuroticism as well as dysfunctional attachment styles displayed a significant positive correlation with relationship-related problems. Traumatic childhood experiences, insecure attachment styles as well as neurotic personality characteristics contribute to increased relationship disruptions in couples. Relevant hormonal and psychosocial parameters in BPD partners should be taken into account when treating females with BPD.