You need either some Charisma or Illusion magic iirc, not that sure tho since I'm currently always able to flirt with it apparently
Edit: Started a new run to test it out and the Flirt Shadow option appreared when I was levelling up Summon and Illusion, rather low levels but I'm not sure which one tho cuz it was sped up and I wasn't paying enough attention. Charisma is only to fuck it