Esther can play serenades to seduce creatures into becoming your familiar.
Once the Temple of Trials has been completed, Esther gains the ability to use Serenade to catch new familiars. This is done by fighting monsters as usual, but sometimes - depending on the creature there's roughly a 4-25% chance that this happens - a bunch of pink hearts will appear above the creature, indicating it's fallen in love with you and is ready to be seduced. Immediately stop attacking it and have Esther perform the Serenade to catch it.
If you get the Jack of Hearts merit award, you'll have a higher chance of being able to serenade familiars.