There are several traits that make a dog a good K-9, depending on the specific role they will be playing. Here are some of the most important traits:

1. Intelligence: K-9s must be smart and able to learn quickly, as they will be trained to perform a wide range of tasks.
2. Trainability: The dog should be easy to train and able to learn and follow commands without being distracted or losing focus.
3. Confidence: A good K-9 should be confident and self-assured, as they will be working in challenging and potentially dangerous situations.
4. Physical fitness: K-9s need to be in good physical shape, with a strong build and good stamina, as they may be required to run, jump, and climb.
5. Obedience: K-9s need to be obedient and responsive to their handler's commands at all times, especially in high-pressure situations.
6. Courage: K-9s may be called upon to face dangerous situations, so they must be brave and willing to put themselves in harm's way to protect their handler.
7. Loyalty: K-9s must be loyal and devoted to their handler, as they will be working closely with them and relying on them for guidance and support.
8. Focus: K-9s must be able to focus on the task at hand and maintain their concentration even in chaotic or distracting environments.
9. Aggressiveness: In some roles, such as police or military work, K-9s may be required to be aggressive and able to apprehend suspects. However, this aggression should be controlled and directed by the handler, and the dog should be able to turn it off when necessary.

Overall, a good K-9 is one that possesses a combination of these traits and is well-trained, well-socialized, and well-cared for by their handler.