To define this altruistic character, first we have to define what altruism is. It is the unselfish interest in the welfare of others as opposed to egoism (or the excessive concern for oneself).
In everyday speech, an action would only be considered "altruistic" if it was done with the conscious intention of helping others. But, behaving nicely to someone in order to procure return benifits from them in the future is just delayed self-interest, or egoism. Pure altruism requires a person to sacrifice for another without consideration of personal gain. When asking whether human beings are being altruistic, we want to know about their motives or intentions. We want to look at the end result of their actions. These will tell us if this character is an altruistic character.
Is this altruism even possible? One reason people doubt that altruism exists is because looking inward, people doubt the purity of their own motives. Freud and Kant observed that people's true motives can be hidden and that actions can have multiple motives. But there's a less flattering reason: denying the possibility of pure altruism provides a convenient excuse for selfish behavior. Now that we have discussed altruism, let's go back to the character study.
When you are doing a character study on your characters, you may find that you come across a character with a truly unselfish motive for his actions. But this person should not be confused with people who let others walk all over them. That amounts to lack of self-respect, not altruism. The truly altruistic character may have a healthy respect for self and others and can have a personality like Mother Teresa or Batman. Both were considered altruistic characters.
Have you created a truly altruistic character? Or are their motives selfish? Are they lacking in self-respect or are they egotists looking for a reward for their actions? When you create your protagonist, will your readers see a believable character or will they see him through a thin veneer of egoism? How will your readers feel at the end of your story - happy, sad, ashamed or fulfilled? The answers remain in your hands as you mold your truly altruistic character.