Guardian angels, or "GAs" as they're sometimes called, typically perform their protective duties towards a single person, sometimes a very small group. GAs are highly regarded, and are reserved to more devout, Angelic-aligned sapients that have earned the honor of being protected by one of Heaven's finest and most talented. It is very common for a guardian angel to be an angel that has protected the same person or group of person for a while, and gained enough experience to reach the level of guardian, then simply continued to protect their favorite regulars.
Should a guardian angel continue surviving and fulfilling their duties, they may eventually evolve once again, and rank up to the next form, the [[seraph]]. Although details and specifics are not known, judging by the relative rarity of seraphsseraphim, it appears that the gap in experience between GA and seraph is much greater than that between angel and GA.