If my boyfriend were to ask me who he is to me, I might respond with something heartfelt and honest, like:
"You are someone incredibly special to me. You are not only my boyfriend but also my confidant, my best friend, and a pillar of support. You bring so much joy and happiness into my life, and being with you feels like home. You've been there for me through the ups and downs, and I cherish the memories we create together. You mean the world to me, and I am grateful every day to have you in my life."