An angel is a spiritual being created by God to be His servant, and also to serve those who will inherit salvation, as Scripture says. However, Satan and 1/3 of the angels rebelled against God and were cast down from heaven, with Satan causing mankind to sin. Though Jesus in Scripture implied that the angels in heaven could not have sex or father children, it is also suggested through what Jesus had said to Satan in "Glorious Appearing" that the angels that fell with Satan fathered children through the "daughters of men" in the time of Noah that produced the Nephilim.
There were angels that appeared throughout the Left Behind books that provided deliverance and protection for the believers in Christ during the Tribulation period and also during the Millennial Kingdom. One of them was named Anis, who appeared during Buck Williams' attempt to smuggle Tsion Ben-Judah out of the country of Israel to the safe house next to the New Hope Village Church, when Buck and Chaim Rosenzweig visited the empty tomb of Jesus Christ after Leonardo Fortunato had incinerated Hattie Durham, and when Rayford Steele was kidnapped by members of The Only Light.
Angels are shown to wield great powers, but have few if any desires or ambitions on their own -- in Kingdom Come: The Final Victory they are described as "not qualified as guests" during the Feast of the Bridegroom at the beginning of the Millennial Kingdom. Powers shown include selective invisibility, enough telekinesis to remotely open cell doors, and the ability to utter a soporific benison that can cause unprepared humans to fall asleep instantly. Presumably, they are impervious or resistant to projectile weapons. In the Bible, they are also shown capable of making limbs go numb with a touch.