This was Ariel-sama and Luke’s plan.
First, by revealing my underwear to him, no matter how dense Rudeus is, he can’t feign ignorance.
I’ll describe the situation while still in my underwear and tell him that I am the Sylphiette from Buina Village.
The girl he likes is in her underwear, add to that, she is his childhood friend as well, using that momentum I confess my love and then he can’t reject me and just like that we'll go to bed.