The Namae Namae 'no Mi' is a Paramecia class fruit that allows the user to steal the names of people and use them to contrl others. It was eaten by Marine Captain Quincy Franklin.
The power of the fruit is that the user can 'steal' the name of a person and use that to control them. Using prior knowledge, if they can write the name down, they can use this to control people as long as they have the name in their sole possession. Taking a name erases the name from the memory of the person. Using this ability, they can force people to do their bidding. It can also be used to steal the name of a named object, such as a weapon.
The fruit will not work if the person lies about their name, or if they lose possession of the name. Other people can also tell the affected subject that name, thus bolstering their memory. Once the name is regained, it can never be stolen again. Also, if it doesn't have a specific name, then it cannot be stolen. The commands are only strongly emphatic suggestions, not mind control. To steal the name, the user must have it told to him by the victim themselves- thus, a name told by someone else won't work unless the victim says it. They also have to initiate physical contact after hearing the name in order to steal it.