I am sorry to disappoint you. There is no such thing.
Learning a self defence that minimizes the pain to yourself and others is one of those things that people seem to asking for a lot but that can never exist. It is like the proverbial wish of eating your cake and having it too...
By definition, martial arts are all about effective application of pain and discomfort.
It is also implied in the practice and practical application of martial arts that you only really use it when facing an immediate and unavoidable threat to your life and/or that of your close ones.
If you should ever find yourself in such a situation, you need to be aware that the attacker has already crossed the unspoken threshold of the social norm and does not give a flying s**t about your feelings or health or well being. He (or she) is out to hurt you. He is ready for this and he will not stop even if you ask nicely not to. This is the territory that is completely out of usual comfort zone of an ordinary person...
The only option in this case is quick and efficient (well, as quick and efficient as you can manage) application of pain and/or discomfort that will disable the attacker (or attackers) for enough time to get out of this situation without further damage.
If you have learned (and mastered) some form of martial art, you have a chance of more control over the amount and the manner of hurt you can inflict, but there is always some pain involved (with implied threat of more pain and discomfort)...
Another concern that I read in your question is that you want to learn martial art without getting hurt in the process. This is impossible.
The process of learning to function efficiently in the situation of conflict involves putting yourself in a similar situation. A lot!
If you want to learn to fight, you have to fight to learn.
As part of the learning process, you will make mistakes that will result in hurting yourself or your training partner. Maybe even seriously. That is the nature of the martial arts practice. You can not completely avoid this — only keep your eyes open to avoid the most serious of injuries. Without this crucial component you might just be better off learning yoga or gymnastics instead. Be prepared to eat bitter, as chinese say it...
Or learn to run. Longer and faster than anyone else.