Saint Agnes, painted by Domenico Zamperi (also known as Domenichino) in 1620.
This is a portrait full of symbolism. Some of the more obvious are the halo at the saint's head and the crown given to her by the putto (the chubby baby with wings, in plural 'putti'). That is the crown of martyrdom handed to the young girl. At her feet is another putto, holding on to a lamb. Lamb is agnus in Latin and often shown as her symbol - it both reminds of her name and the lamb of God, all wrapped into one neat package. The palm-leaf, also held by the flying putto, is another sign of martyrdom, and of martyrs overcoming death.
The dress Saint Agnes is wearing is not the fashion of 1620 but how people in the 1620's imagined that people living in antiquity might be clothed. It was important to show that this was not a modern lady - but it was also important to show that she is a really important figure and therefor she is adorned in the style of royals. Note that her mantle is lined with ermine, something that was usually reserved just for royalty.
Saint Agnes is one of the popular saints, and has been so both in the East and the West. Her legend (written in the fifth century) tells that she died sometime around 305, at the tender age of thirteen. She had taken a vow of chastity to please the Lord and refused to marry - for this she was executed by having her throat cut. Later on this legend got expanded and included how she was put in a brothel so that she could be raped and her chastity thereby taken from her (a common theme in female saints' legends - and it always back-fired). But a man who saw her naked went blind on the spot.
Her fest-day is on 21 January and in Rome lambs whose wool are used to make the archbishops' pallia are blessed.