Talk with AIKO about recruiting Serena.
The AI will suggest you focus on others first, because she doesn't have any immediate plans involving her.
Requirements:
Finish Bob's Recruitment
Finish Alex's Recruitment
After finishing both events, you'll get a random work event where Bob and Alex bring in a "moving crate".
Alex will be inside the crate, bound and gagged.
Alex's behavior depends on whether or not you are AIKO's avatar.
You will be presented a choice; send your ex to become a slave or have her join AIKO's growing inner circle.
If you choose to enslave Serena, she will be carted off. Alex suggests sending her to Primrose.
Otherwise, Serena will be released and put to work with Alex in IT.
If you are not AIKO's avatar, Serena will quickly assert her dominance over you.