Day one is a term stolen by basic women (and men) for their "insta" captions. It references a person that is close friend. The term is really referring to someone that has been there since your day one and therefore should only be used by twins who have a literal day one.
Your ride or diebestie, someone who is down to ride and by your side 24/7. Doesn't ask for anything but your loyalty, but would give you everything they have.