The one who is always there, there for us through every single up and down of our life.
The one who is always right there, riding the coaster with you, squeezing your hand tight on all the biggest, scariest hills telling you, "You'll be okay, you'll make it through. You can do this." You know that that person will never hop off the coaster at a stopping point to find an easier ride; that one person will stick with you because they want to. This person, they complete you, they bring the biggest smile to your face and make your life worth living. You know that they will always be there for you, no matter what.
Twenty-four hours a day, seven days a week, and all three hundred sixty-five days of the year.