History; his painful past

Neil Dylandy was born on March 3rd, 2283 AD (making him 24 at the beginning of the series) to two loving parents in Ireland, which is under the AEU. He had a sister name Amy, and a twin brother name Lyle. This happiness came to an end when a terrorist group attacked and killed his parents and sister, leaving the twins orphaned. This event in his childhood left Neil with a hatred of terrorists and war. It's not clear when he joined Celestial Being, or how, but no doubt he hoped Celestial Being could give him the power he needed to avenge his family. His twin, Lyle, is shown briefly in an early episode, visiting their family's grave while Neil hid from sight. It would seem the two went their separate ways and do not keep in touch (especially since Neil is suppose to keep his activities hidden).

As it turns out, Setsuna F Seiei and Lockon Stratos had shared a connection in their past neither of them realized, until Johann Trinity, a rogue Meister, reveal the top level information. For Setsuna's brief history, see here. Neil's family was killed by the terrorist group, KPSA, of which Setsuna was once a member. Even with this, Neil did not direct his anger toward Setsuna, but instead focused on the fact both of them were willing to do all they can to intervene in warfare, here in the present. KPSA is led by a man by the name of Ali Al-Saachez, who was well deserving of Setsuna's hate. This man would later be Lockon's Achilles' Heel.

When Terrorism is mentioned, Lockon gets emotionally involved, showing this subject to be his weak spot. It is something he cannot stay objective about. Of course this means he doesn't enjoy the fact Celestial Being is often regarded as a terrorist group, through their armed interventions.