Is the Devil a fallen angel? If not, who or what is it?
The devil is not the rebellious angel of Christian mythology.
Angels cannot sin or die because they have the same divine nature promised to us.
2 Peter 1:4 Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
Luke 20:36 Neither can they die any more: for they are equal unto the angels; and are the children of God, being the children of the resurrection.
The devil is the personification of the propensity to sin within man, both individually and collectively (as in governments).
We all have the 'devil' within us.
Mark 7:21 For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, 22 Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: 23 All these evil things come from within, and defile the man.
We are tempted to do evil by our own lusts, not by a supernatural entity.
James 1:14 But every man is tempted, when he is drawn away of his own lust, and enticed. 15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
May I suggest that you read: ‘Do You Believe in a Devil?’ here: http://www.thisisyourbible.com/index.php?page=library&task=show&mediaid=162
And ‘Basic Bible Teaching - The Devil’ here:
I hope you find this helpful.