Specifically, I am looking to describe a person whose only purpose is to help others, not caring about what happens to himself or herself (physically or otherwise), though without actively seeking pain.
proceeding from or revealing generosity or nobility of mind, character, etc.: a magnanimous gesture of forgiveness.
A magnanimous person is someone whose generosity of spirit is large. A person willing to forgive an evil deed, demonstrating nobility and generosity of mind to his/her enemies.
Always thinking of what other people need or want and being careful not to upset them He was always kind and considerate.
Disinterested: free of self-interest:
2 : free from selfish motive or interest (see interest entry 1) --mirriam-webster.com
Although it is perhaps more commonly used to indicate a lack of selfish bias in a decision, it is also correctly used more specifically as the word you asked for:
Different scientists from different countries made special researches and in 2013 they came to the same conclusion: we can really live longer if we start to help other disinterestedly. --livecustomwriting.com
Your works, he said, testify to the priceless value of gratuitousness and disinterested aid in a world in which profit and business seem too often the only points of reference. --linguee.com
Wealthy investors likes Kennedy and his partners try to portray themselves as disinterested parties who are trying to help the environment and our country, but they have a stake in the game. --huffpost.com
In Mahayana Buddhism, a bodhisattva is "a person who is able to reach nirvana but delays doing so through compassion for suffering beings."
Nirvana is (in Buddhism) "a transcendent state in which there is neither suffering, desire, nor sense of self, and the subject is released from the effects of karma and the cycle of death and rebirth. It represents the final goal of Buddhism."
"A person whose only purpose is to help others, not caring about what happens to himself or herself (physically or otherwise)" is a bodhisattva.