How do you do that is there any prerequisiteYes this is netorase.
You get magic semen / mana for loaning her out that you can trade in at the shop.
You can send the knight to the brothel, loan her out to drunks at the bar, students for pocket money, or men at the street at night for a pittance amount of gold.
You can also lend her to another party member for automatic mana generation daily.
The only NTR end as mentioned before is blacksmith, and from what I've seen it was because she thought it would bring you the greatest happiness due to your "condition".
Also one/two swinging scenes where you can trade partners with the shopkeeper you trade materials in to.