Fujiwara

"Fujiwara", nicknamed "Sneering Bastard" by Kyon, is a time traveler, like Mikuru Asahina, but from a rival faction.
"Fujiwara" is obviously a false name, that of an ancient famous Japanese family.

In the β timeline, Asahina said of him that his motives weren't necessarily bad, but she didn't have information about him from the future. She confirmed that he was from her time period and used a TPDD to travel to the present time plane.

Fujiwara seems to be under some of the same restrictions as Asahina, for instance when he was angry with Kyon he threatened to "classified information his classified information" rather than giving a threat Kyon could understand.