Ideally...
1. Adequate athleticism - he'll need to have enough speed/range to cover the scope of assignments he'll draw, decent vertical to contest jump balls with receivers, excellent balance to stay on his feet while adjusting to sudden changes of direction, good change of direction quickness/speed, the ability to get up to game speed with one or two steps, functional strength to stack & shed blockers, wrap up tackles, and out wrestle bigger receivers for control of the ball.
2. Field Vision - excellent field vision combined with very good football knowledge to see, process, recognize, and react to a developing situation.
3. Communications - excellent communications skills on and off the field to coordinate the secondary with the front seven, discuss & listen to coaches and fellow defenders, and take the game plan from the classroom to the practice and game day fields. EDIT: The ability to encourage and teach others without belittling falls in this category.
4. Attitude/Mental Discipline/Focus - He needs to believe in himself and his teammates, communicate that belief, and have the mental discipline to learn from mistakes (his and others) and move to the next play with no recriminations or emotional/mental garbage to distract him from his job.
5. He has to like contact - It is a plus if he can hit hard, but it's more of a plus if he can absorb a hit and be energized to come back and do it again and again and again.
6. Durability - This is a combination of conditioning and the desire to maintain the conditioning necessary to play at a high level throughout the entire season.
7. Hard Worker - Not just the work to maintain his skill set, but the work to improve his weaker areas and learn new skills.
8. Drive - he's got to want to succeed in each of the areas listed above and in every challenge he faces. There has to be a competitive and disciplined hunger to be the best.
9. He has to thrive on chaos, not be a source of chaos (except to the other side), but someone who rises to the challenge of a chaotic situation and literally grows stronger and calmer amidst the mud, and the blood, and the tears.