Best Friend's Brother

The romance trope where the heroine falls for her best friend's older (or occasionally younger) brother, often a figure she's known her whole life but only now sees as a romantic prospect, creating built-in family and friendship tensions.

Readers love the built-in intimacy and history between the characters, plus the delicious secrecy and risk of betraying a friendship for love. The brother often brings a protective or possessive edge that contrasts with his familiar, almost family-like role.