The Ambling Pirate is a lone Sinner character that is eight times the level of the enemies on the map. He can be found on any floor of the item, and you'll probably get annoyed by him after a while, as he has the highest chance of appearing than all other Pirates. Due to the high level, he is usually hard to beat, or thanks to a high speed, hard to hit at all in the deeper floors of an item.
If he gets in your range, if at a lower level, it may be best to leave the floor as quickly as possible to avoid deaths caused by the Ambling Pirate.