From the walkthrough: - After capturing Lind in the last vanguard battle with her, you can advance her event by speaking with her in the prison, and later at the meeting room in the sleeping chamber area. When you allow female soldiers in the finances and policy screen, you'll get an event where the manticore Mia is recruited and send to the repaired barracks of the vanguards. Talk with Mia in the barracks when you have at least 1 captured blue and green slimegirl, and the events with Jackie and Lexi advanced to the point where they're standing at the entrance of the monsterhall in your castle, to recruit Mia as a squad leader. Instead of equipment, her squad can change the monstergirls that she uses to fight with, which requires that you unlock their scenes first.
For the false love achievement you need to give a love potion to a girl with so low relationship to Ryen that it'll have a negative instead of a positive effect.