Kagome Yuu is a sealmaster of unknown national origin. (His given name, 'Yuu', is only known through OOC information. He has not actually revealed it to the party yet.) He suffers from extreme paranoia and anxiety, especially with respect to sealcraft, but despite this was convinced to teach the art to Hazou. According to Inoue Mari's assessment, his mental health has been making progress since he joined the team. In game-mechanical terms, Hazou estimates Kagome's skill level with sealing level is at least 15 and probably no higher than 25. (Note this answer was given before The great rebalancing so probably no longer accurate)
Kagome joined the party after the initial four members infiltrated the Liberator's camp in the land of Iron on a mission for Jiraiya and escaped it, bringing along Akane. First contact occurred before that mission, when the initial four members investigated rumors of a 'Black Hunter' near a small village in Iron where they were staying.
We know little of Kagome's personal history, though he has indicated he spent five years in a cryptology department and had "Black clearance, but only grade two", as well as a period of unspecified length being forced to manufacture seals in a "seal factory" prior to his escape into the isolation of the woods of Iron, where he says he stayed for fifteen years.