Complete the Rival storyline
You'll see a random event during work when a special crate is dropped off.
Once you take the crate to Ms. Antlers, she'll reveal that it's none other than the rival's former SISSI secretary. Ms. Antlers wonders how to use this expected 'asset'.
You must then choose whether to keep her as a SISSI secretary or to reposition her towards management.
You can choose either option, but the secretary will be disappointed if you choose to keep her as a submissive secretary.
You'll see a short scene at the company canteen some time later where you'll see the secretary in her new role.