His name was Abu Ubaida ibn al-Jarrah and he was the ninth person to accept Islam. Participating in all of the battles led by Prophet Muhammad, during the first two caliphates he was the commander of an army in Syria and Iraq.

An incident occurred at the battle of Uhud which explains the title of this article.
Abu Bakr said, “The Prophet was injured in the battle of Uhud. A couple of links from his own helmet had pierced through his forehead, which was bleeding. I ran towards him, and then suddenly, what I should see but a friend
running towards him from the east. He raced him before me and I saw that it was the self-sacrificing Abu `Ubaidah bin Al-Jarrah.

He requested that I should give him the honor of removing the links from the helmet. With great care and at the same time with great force, he lost a tooth in the process. Then without bothering about the pain, he must be suffering, again he held on to the other link with his teeth and exerted force. As the other link came out, he lost another tooth as well. Thus two of his teeth were sacrificed, but his beloved Prophet was found relieved and comfortable.

It seemed as if he looked even more handsome and graceful than before with the loss of his two teeth because it was done for the love of the Prophet. All the other companions used to look at him with envious eyes. There the only criterion for judging the worth of a person was the extent to which he was willing to sacrifice himself for the Prophet.”
[Ref: Ghandanfar, Commanders of the Muslim Army]

NB. One of the customs of the Arabs in that era was to have a gap between their front two teeth. It was considered a sign of beauty. The comment in the quote about him looking more beautiful doesn’t relate to this custom of having a gap between one’s front teeth, but more because of the reason, it was done i.e. to help remove the links from the helmet.