"Become my only one" can be translated as "become my sole/exclusive/unique/paramount/preeminent one." The phrase is a romantic expression used to express a deep connection and commitment between two people.
Here are some example sentences:
"I want you to become my only one, to love and cherish for the rest of my pfe."
"You already have a special place in my heart, now I want you to become my only one."
来源：这是一个常用于表达爱情的浪漫的表达方式。