It is difficult to pinpoint major roles in the nature of any given deity, but I think that Guardian is one that is well suited to Apollon (and likely by Artemis too for similar reasons) because despite all of his destructive tendencies to a purpose in nature, many of his functions within the natural world and cosmos are that of a divine guardian figure. I am not speaking of the popular idea of a “guardian angel”. That is actually more in keeping with ideas of personal (rather than household) agathos daimons. Instead Apollon as Guardian is less concerned with personal guardianship in the sense of personal welfare, and acts in more in a general fashion to maintain harmony and integrity of the whole.
As such we see Apollon in the Orphic hymns as a conductor of the passage of time and the seasons, keeping the harmonic movement of bodies in place (as also noted by Socrates in Plato’s Cratylus). He is a guardian operating and maintaining this delicate balance of time passage, of beginnings and endings and new beginnings again. He is the mythic grandson of Koios, lord of the axis and so too Apollon himself manifests at the axis of the heavens by his birthright, and at the axis of all things. This harmonic turning and balance is also connected to his affluence over human health and wellbeing, of both the body (healing) and of the soul (purification) for which his name too means “Washer”. The wholeness of one and the integrity of their self is a microcosmic reflection of what he is doing in the cosmos and therefore he is rightly petitioned for this aid.
In his domestic function we can clearly see him manifest most clearly in the sense of the Guardian god, not only as a god of the gates (Agyieus) and doorway (Thymbraeus), and the Noumenia when the new month enters with the rebirth of the moon through the gates of the heavens to slowly begin her growth to maturity. This too touches on the entrance of new life in general into the household, during which Apollon (and Artemis) is the protector of the new babe and is so honored when the baby passes the first seven critical days of infancy whole and intact. Thus on that day the parents and household honors Apollon and Artemis as protective caregiving gods and the baby is only then named. Then at the end of life Apollon acts as a guardian again at the cemetery in which the deceased is place under his care within the womb of the earth for the 30 (or 40) days that it takes for the bonds between the body and soul to decay, protecting the tomb and the soul until it is retrieved by Hermes. Naturally between these two extremes Apollon acts in the transitions of life, at every stage exercising his vigilance and care. Thus when boys pass into youths he is there, when youths pass into manhood he is there, when the marriage hymns are sung and new families begun he is there, when the initiates enter into the mysteries he was there, and when death takes old age he too is there.
Thus we find him at the harbors, being the safe haven for ships, sailors and travelers by sea (which was largely unknown, hazardous and often enjoy some symbolic connection with the passage to and from the underworld). We find him in the liminal zone of the mountains looking upon all that rests beneath his seat, at the cities and villages nestled in the valleys. He is the guardian too of the altar of Hestia upon which burns the flames. We also find him as the oracle, giving careful measure in his divine messages, to reveal no more than what he is by necessity constrained to reveal, guarding as from the dangers of knowing too much and hindering our own growth. And he too is the Orphic guardian of the gates so to speak of exiting out of the mortal cycle (for which Hermes who receives the soul would move it to another abode rather than that of Hades) and he provides the measured test to all those who approach his gate.
It as Destroyer (and Artemis too as the Uninjured/healthy) that he is best suited to perform the acts of the guardian even as Hermes is so often understood as the Guide of Souls.