Is talking about an unknown person to a female friend considered backbiting, for which one will be held accountable, and is seeking forgiveness and supplicating for the one who was backbitten sufficient to erase the sin without their forgiveness?

Narrating a story or mentioning an incident that happened to someone the listener does not know is not considered backbiting (ghibah (definition)). A Muslim must avoid backbiting in any form, as it is one of the major sins. If backbiting occurs, one must repent and seek forgiveness from Allah, and it is more complete to ask for forgiveness from the person who was backbitten.