Unaggressive means peaceful or agreeable. If your dog is sweet and friendly, you can describe her as unaggressive.
Someone who's unaggressive might fight, but never without being provoked. Generally, unaggressive people are mild-mannered peace lovers. Unaggressive things are equally gentle, like the unaggressive tone of your letter to the editor or the unaggressive background music you like to play when you're doing homework. Add the prefix un- ("not") to aggressive ("likely to attack") and you get unaggressive.