For Zayn Malik, his daughter Khai is a big part of the story of his life.

In fact, the One Direction alum considers the 4-year-old—who he shares with ex Gigi Hadid—to be "the most important person" in his personal journey.

"I love you more than words allow me to express, beyond proud to call you my daughter," Zayn, 31, wrote on Instagram in a birthday tribute to the toddler Sept. 19. "Grateful for every second I get to spend next to you, as you become the incredible person I know you already are."