When you talk with AIKO about recruiting Antlers, you will learn that she drinks a special tea to keep her loyal to SISSI. You'll need Dr. Twig's help to counteract this drug, so she can change sides to AIKO while keeping her usefulness to the new order.
A fancy tea set will be sent to you to mark the occasion.
Visit Dr. Twig's office and ask about an antidote to the Antler's drug.
After some discussion, you will receive a bottle of antidote. Dr. Twig also warns that you will have to be on your toes to handle Antlers after the drug is administered.
Now you can take the spiked tea set and visit Antlers.
Antlers will express some annoyance, but she will accept the spiked tea you prepare.
After Antlers drinks the drugged tea, you can then make your pitch.
After some discussion, Antlers will be on board. But she will ask what your role will be if she changes how things work.
If you are AIKO's avatar, you can ask to be her equal or her superior.
You must have [Dominance >= 20] and [Smarts >= 20] to become Antlers' superior.
If you don't have these stat requirements or choose to kneel after making your play, then Antlers will remain in charge. But she will compliment how far you've come.
Otherwise, you will submit and remain at her feet.
If you opt to serve as Antler's submissive, then she will commemorate the new goal by punishing you.
After the scene, Antlers will dismiss you to recruit more allies.