Yes we still believe in the devil. Not the cartoon with horns, tail and pitchfork, but a purely spiritual creature. And this is what the catechism underscores is that he is merely another one of God's creatures, who by his own free will turned away from God.
There are three words in the scripture that underscore the core of our understanding. The first two are the names we use
Satan is the Hebrew term, the accuser, the adversary. Think legal term. On your side is the advocate; on the other side the accuser. If the Holy Spirit, the Paraclete is there to tell you all truth, the Satan is there to tell the lie.
The Greeks use a different metaphor, Διαβολος- diabοloς, devil. It literally means to throw across. Think about strolling down the road and someone throws something across your path. In short, it's things that trip you up.
The third word is temptation. In both Greek and Latin it refers to testing.
We all experience these voices. The voice that keeps us from truly feeling forgiven even when we have just gone to confession. The voice that distracts us when we try our best to pray. The voice that rationalizes not doing a good deed. If we can get past the caricatures, we all have experienced Satan, the Devil, the Tester.
The catechism describes it this way
He is only a creature, powerful from the fact that he is pure spirit, but still a creature. He cannot prevent the building up of God's reign. Although Satan may act in the world out of hatred for God and his kingdom in Christ Jesus, and although his action may cause grave injuries - of a spiritual nature and, indirectly, even of a physical nature - to each man and to society, the action is permitted by divine providence which with strength and gentleness guides human and cosmic history. It is a great mystery that providence should permit diabolical activity, but "we know that in everything God works for good with those who love him."
And like Jesus we can choose to turn away from sin.