While this angel goes unnamed, we can deduce a little bit from the context. The verse mentions that "he called with a loud voice to the four angels." This gives some indication of authority over the other angels.
Rev. 12:7 says "Michael and his angels fought against the dragon". Show that Michael the archangel has authority over the other angels. So we can deduce that the angel of chapter 7 is Michael of chapter 12.
As to why he is ascending, there is much speculation as to what this means but nothing substantial has been acknowledged.