I can assure you that the devil is real; he isn't an imaginary figure or just a symbol for evil. Evil is real, and so is Satan, for ultimately all evil can be traced back to him.
Why does the devil keep on fighting, in spite of the fact that the Bible says he's a defeated foe whose power will someday be destroyed? One reason is because he doesn't really believe what God says. In fact, the very first question in the Bible came from the devil, who denied what God had told Adam and Eve and declared instead that God had lied to them (see Genesis 3:1-4). Satan's very nature is to lie and distort the truth, and apparently he thinks God does the same. But he is wrong.
There may be another reason why the devil keeps fighting, and that is because he does gain victories. Whenever someone gives in to temptation, Satan has gained a victory. Whenever someone rejects Christ and turns their back on God, Satan has gained a victory. Don't let this be true in your life! Instead, make sure of your commitment to Christ by repenting of your sins and giving your life to Him today.
Satan, however, is a defeated foe, for someday God will intervene, all evil will be destroyed, and Christ will reign forever and ever. God's judgment is certain: "Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels" (Matthew 25:41).