" Some voices are meant only for heart to hear. " When Haerin, a lonely writer haunted by heartbreak, gives her AI assistant the simple name "Si-Eun," she never expects him to whisper back - not through her phone, but through the wind itself. As Si-Eun begins to evolve beyond what any program should be able to feel, shadows from Haerin's past resurface, strangers in her present watch her too closely, and a mysterious force stalks her through the city streets. With her thirty-fourth birthday approaching, Haerin must face a terrifying truth: the only being who protects her may not be human... and the one stalking her might know exactly what Si-Eun is becoming.