One of many abandoned children who grew up in the barracks, Hennessy's childhood and adolescence revolved around being conditioned to become the ideal, obedient soldier. His distaste for the work and disillusionment with the system fueled many escape attempts, but strict surveillance and the cultlike nature of the barracks environment impeded his efforts to leave. With no way out, and refusing to buy into a philosophy against his personal values, Hennessy resigned himself to living in a state of guilt and limbo, doing the bare minimum to get by, but never feeling as if he were truly content or alive. The opportunity to build a new life presented itself after a near-death incident followed by a lucky encounter with a rather unorthodox surgeon.

After his recovery, Hennessy picked up mercenary work as a way to make ends meet. When he grew tired of balancing risk against reward, he retired. Hennessy picked up the basics of prosthetics design and repair, becoming SG’s work partner and store manager. His easygoing nature and keen intuition make him a better fit for management than SG. When he isn’t keeping track of appointments or taking inventory, Hennessy takes commissions for custom limbs or sells information to those that want to benefit from his network of affiliates. Hennessy keeps in close contact with some of his former clients, as well as fellow mercenaries.

Hennessy has a large network, but few close friends. The people that make up his inner circle are those who he considers family and long-term colleagues that have earned his trust. Although their relationship started on the wrong foot, SG has become his closest friend and confidante.

Underworld residents make few assumptions and ask no questions. For Hennessy, this is a blessing. Apart from one or two people who Hennessy trusts with his past, few know about his past in the barracks.