Guardian Angel

No angels are true "guardian angels" in the modern sense. They can be assigned to a person but can switch assignments once the protection is no longer needed.

Escorts
Angelic escorts can be angels of any rank, and more important people will have more escorts. People or places of great importance will be assigned a Cherub or two. Cherubim are very strong angels, immovable forces of defense, each one able to take on many demons without faltering but, like others, they will not stay with one person for the extent of that person's life, unless the situation warrants it.

Angelic Partners
Keruvim begin their training and will spend much of their life with a "partner" who will travel with them and protect them  whenever they can be spared. Partners usually stay with the same keruv for the extent of the keruv's life (or the angel's, in the case of defeat) but will not be bound to follow them around; older, more experienced keruvim often choose to travel without a partner.