Lloyd and Rixia are not the pair lead of Zero or Ao. The leads of Zero are the SSS (Lloyd, Randy, Tio, Ellie) and Ao is the same with a few more characters joining the SSS (and leaving and joining again lol)
However, Rixia does join your party periodically in Zero (in disguise) and permanently joins your party late in Ao.
Falcom is quite liberally pushing Lloyd and Rixia as a couple, they stand the closest together in their combo craft compared to everyone else, her confession scene is probably the most personal (with a very heavy main quest designed around Lloyd saving her from herself, though it's mostly Ilya who ultimately does that), and she is chosen to appear in the CS2 Epilogue next to Lloyd with all the suggestive dialogue together... heh. But I don't particuarly like that, because anyone who's played Zero and Ao know that Ilya and Rixia are the true item, and that Randy should've been the partner to Lloyd in CS2 instead.