Yasashii Knight Wa, Mou Yameru. ~Gokuama Osananajimi No Dokusen Yoku Ni Zuppuri Hameraretemasu summary:

Yui is a humble, cute, and popular salesperson at a department store jewelry shop. There are countless men who have feelings for her, but there are secret rumours that at closing time a "knight" comes to see her off. Her "knight", an elite trading company employee, conceals his love for Yui for some reason, and watches over her as the closest person in her life. However, after the appearance of a certain male customer, this "knight" becomes her controlling, blindly infatuated boyfriend...? "It may be embarrassing, but I want to see all of you, Yui... Will you stay like this, and be mine, and mine alone?? She melts for her childhood friend turned lover... Why? Even though it's a little scary, why is it she wants it even harder...!?