Lucien is Cetra, much like Aerith is. However, he is from a subsect. If Aerith's ancestors are the spiritual guidance and the philosophy, Lucien's was the protective and solely militaristic arm of that philosophy. While most Cetra were not warmongers, that did not mean they wouldn't-or couldn't defend themselves. Called a Bellator, he is the result if the planet created something akin to humanity's SOLDIER. He is infused with the lifestream, however because of his physical abilities being enhanced, his ability to hear the planet and be guided is diminished. This is theorized to be a way the two races relied on each other.
Lucien had been trapped in a spell that his mother had cast 2,000 years ago. She wanted to see to it that after the Cetra were driven to brink by Jenova, the world would not be without them entirely so she had hid her son. After Lucien emerged from the spell, the world had drastically changed since 2,000 years ago. Lucien first set his sights on destroying Jenova, but in finding out about Infalna and Aerith, he joined ShinRa to learn that the Cetra were all but gone. So he watched them both after becoming a member of SOLDIER, becoming a sort of black sheep amongst his peers in 1st Class because of his alarming physicality and disregard for the use of magic or materia.
Lucien had been trapped in a spell that his mother had cast 2,000 years ago. She wanted to see to it that after the Cetra were driven to brink by Jenova, the world would not be without them entirely so she had hid her son. After Lucien emerged from the spell, the world had drastically changed since 2,000 years ago. Lucien first set his sights on destroying Jenova, but in finding out about Infalna and Aerith, he joined ShinRa to learn that the Cetra were all but gone. So he watched them both after becoming a member of SOLDIER, becoming a sort of black sheep amongst his peers in 1st Class because of his alarming physicality and disregard for the use of magic or materia.