The following account of her life and martyrdom is taken from Catholic Online:-
"St. Agnes was a Roman girl who was only thirteen years old when she suffered martyrdom for her Faith. Agnes had made a promise, a promise to God never to stain her purity. Her love for the Lord was very great and she hated sin even more than death! Since she was very beautiful, many young men wished to marry Agnes, but she would always say, "Jesus Christ is my only Spouse."
Procop, the Governor's son, became very angry when she refused him. He had tried to win her for his wife with rich gifts and promises, but the beautiful young girl kept saying, "I am already promised to the Lord of the Universe. He is more splendid than the sun and the stars, and He has said He will never leave me!" In great anger, Procop accused her of being a Christian and brought her to his father, the Governor. The Governor promised Agnes wonderful gifts if she would only deny God, but Agnes refused. He tried to change her mind by putting her in chains, but her lovely face shone with joy. Next he sent her to a place of sin, but an Angel protected her. At last, she was condemned to death. Even the pagans cried to see such a young and beautiful girl going to death. Yet, Agnes was as happy as a bride on her wedding day. She did not pay attention to those who begged her to save herself. "I would offend my Spouse," she said, "if I were to try to please you. He chose me first and He shall have me!" Then she prayed and bowed her head for the death-stroke of the sword.
When one reads the tales concerning the sexual exploitation of young girls, that the popular press seem to ever-more-frequently publish, thoughts must turn to the example of Saints like Agnes, herself only a teenager, who preferred death for Our Lord's sake to a perhaps easier option of succumbing to the gifts and promises of a seducer. Let us pray that she might be able to guide and inspire any youngsters who, today, are facing coercion to yield their virtues for the sake of some transitory presents, and as an Angel protected Agnes, may their Guardian Angels come to their aid.