The Lingering Will is the lost mind of the Keyblade wielder, Terra, residing within the man's discarded armor after his body is stolen by Master Xehanort. It is a briefly playable character at the end of Terra's scenario in Kingdom Hearts Birth by Sleep.
Much like the case of Xemnas's Organization XIII title, the Lingering Sentiment's official English name has yet to be revealed. His name, "Lingering Sentiment", is an approximate translation from his Journal Entry in Kingdom Hearts II Final Mix, "Todomarishi Shinen", and he has also been referred to as the "Fixed Sentiment", "Fixed Feelings" and "Lingering Will". His name, "Lingering Will", is based on the name of the force-field it creates, "Will's Cage".
Tools of the Trade
......I have nothing left to trade other than my Hatred against Xehanort....