Lei Xiaofei wasn't a member of the Zhen Clan or the Zhu Clan. He belonged to a neutral force.
The founders of the Hundred Poisons Sect were the Zhen and Zhu Clans. Later on, they slowly recruited many people with spiritual roots in the secular world, and gradually formed a force independent of the Zhu and Zhen Clans.
However, the Zhen Clan and the Zhu Clan still held the power of the Hundred Poisons Sect and often selected outstanding disciples from the various surnames to join them. However, there were also many cultivators from the various surnames who were unwilling to join the two clans. Lei Xiaofei was one of them. He was also an outstanding figure among the various surnames and was responsible for guarding the first two floors of the stone tower of the Book Collection Peak.