Amongst other things, Saint Agatha is patron saints of martyrs so if you don't like gruesome things look away now.
Agatha lived in Sicily in the 3rd century AD. She was a Christian who had dedicated her virginity to God. Unfortunately the Roman Prefect Quintianus pursued her; when she rejected him, he persecuted her for her Christianity expecting her to give in when threatened. When she didn't, he imprisoned her in a brothel for a month. After this had no effect he then had her tortured. Amongst the tortures her breasts were cut off. St Peter healed her, but eventually she died.
Francisco de Zurbaran
Amongst other things, Saint Agatha is patron saint of wet nurses, which as I've said before is not a perfect fit. For a Saint Agatha costume I can only suggest recreating the common but disturbing image of her carrying her breasts on a tray. That's definitely scary enough for Hallowe'en.
 If I weren't 3 and a half days behind I'd note that both traditionally and officially, the manner of death is one of the details taken into account when someone is canonized, which is why martyrs feature prominently, but I should probably spend the time looking up another saint.
 This method of seduction is not endorsed by Night of the Hats.