In verses 1-3 we learn that Daniel was mourning and fasting and praying for “three whole weeks” because of a vision he had received earlier. In verses 4-9 he received another vision of a glorious man clothed in linen, which left him drained of all strength.  He heard the man speaking though he could not understand him. Now let us read verses 10-14:

“Suddenly, a hand touched me, which made me tremble on my knees and on the palms of my hands. And he said to me, ‘O Daniel, man greatly beloved, understand the words that I speak to you, and stand upright, for I have now been sent to you.’ While he was speaking this word to me, I stood trembling. Then he said to me, ‘Do not fear, Daniel, for from the first day that you set your heart to understand, and to humble yourself before your God, your words were heard; and I have come because of your words. But the prince of the kingdom of Persia withstood me twenty-one days; and behold, Michael, one of the chief princes, came to help me, for I had been left alone there with the kings of Persia. Now I have come to make you understand what will happen to your people in the latter days, for the vision refers to many days yet to come.’”

I believe you are right in thinking this was an angel speaking to Daniel, though some have thought that this was the “preincarnate Christ” because of the description of the man given in verses 5-6 (for it is very similar to the description of Christ in Revelation 1:13-16). But it is doubtful that Christ, who is not only the “Son of Man,” but also the “Son of God,” would have needed help from the angel Michael. We know that Michael, who is called “one of the chief princes,” is the chief “guardian angel” that protects the nation of Israel (see verse 21; 12:1). Whoever this angel was, he was delayed from coming to Daniel by “the prince of the kingdom of Persia.” This “prince of Persia” must also refer to an angel, since he was strong enough to detain the angel who finally appeared to Daniel. No doubt he is a “fallen angel” among the angels that followed Lucifer (who became the Devil and Satan…see Revelation 20:2 with Matthew 25:41) in his rebellion against God. In this passage God is allowing us to see what we cannot see with our physical eyes…the “spiritual warfare that is going on in the heavens above us.” Of course, this warfare is also connected with the earth, for the fallen angel must have been assigned to the nation of Persia by Satan and thus he was “the prince of Persia” who was seeking to influence that nation for evil.  It is just a matter of time and Michael and his angels will do war with these angels for the last time and they (the Devil and Satan) will be cast out of heaven to the earth (see Revelation 12:7-9).

Now as to your question, “Who is the angel that fought against the prince of Persia along with Michael?” I am inclined to believe it was none other than Gabriel, the “messenger angel” who had come to Daniel in previous visions to give him understanding (see Daniel 8:15-16 & 9:20-23). Like Michael, Gabriel was one of God’s “holy and elect angels” that was assigned by God to serve the nation of Israel. I would encourage you to read more about Gabriel’s service as a “messenger angel” to Israel in Luke 1:5-20. I should say that even though I “think” this was Gabriel, we cannot “know” for sure that it was him since his name is not mentioned like it was on the other three occasions (see Daniel 8:16; 9:21 and Luke 1:19).

If we were to read on, we would see that the angel comforted and strengthened Daniel (verse 19). His last words to Daniel teach us that the angelic warfare in heaven would continue and another fallen angel would enter the battle as well. Verse 20 says, “And now I must return to fight with the prince of Persia; and when I have gone forth, indeed the prince of Greece will come.”  Again, these “wars in heaven” are hidden from our eyes, yet they are just as real as “wars on earth” and they have a real effect on the nations on earth as they seek to keep men in bondage to sin and blinded to the gospel of Jesus Christ (see John 8:30-47 and 2nd Corinthians 4:3-4).  (DO)  (571.5)