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.
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.