Angels are guardians of God’s people (For he will command his angels concerning you to guard you in all your ways ~ Psalm 91:11). The Bible simply says that God sends angels to watch over us. Someone has said that angels may play zone defense rather than man-to-man defense as they protect people from harm.
Matthew 18:10 (I tell you that their angels in heaven always see the face of my Father in heaven) speaks of children’s angels, but does not say that each believer has just one guardian angel. God can send his angels where he pleases and can deliver an unbeliever when it serves his purposes, but there is no general promise of guardian angels to unbelievers (Are not all angels ministering spirits sent to serve those who will inherit salvation? ~ Hebrews 1:14).
Although angels can take on different forms, including human forms, they are spiritual beings (without flesh and bones) that God created during the six days of creation. Shortly after creation, some angels rebelled against God and became evil angels or demons (God did not spare angels when they sinned, but sent them to hell ~ 2 Peter 2:4). The chief of the evil angels is the devil, Satan. Please note that angels are not the spirits of deceased people but are personal beings that God created at the beginning of time.
Angels serve God in various ways. God sends angels to watch over his people and protect them from physical harm (For he will command his angels concerning you to guard you in all your ways; they will lift you up in their hands, so that you will not strike your foot against a stone. ~ Psalm 91:11,12).
However, angels are more concerned with spiritual matters than physical matters. They have served as messengers at key times in history and continue to play a part controlling the forces of evil that seek to destroy God’s church. Angels will also accompany Christ when he returns at the end of the world (When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. ~ Matthew 25:31).