Sub-Commander Thei is the Romulan officer who appears beside Commander Tebok when the Romulans end decades of isolation and confront the Enterprise-D at the Neutral Zone.
Thei appears in The Neutral Zone. When the Enterprise-D investigates the destruction of Federation outposts along the border, a Romulan warbird decloaks in front of it, the first face-to-face contact between the two powers in more than fifty years.
Thei and Tebok speak for the ship. Thei is the more openly hostile of the pair, making plain his contempt for humans, and Tebok has to rein him in. Between them they reveal that Romulan outposts have been scooped away in the same manner.
The two sides agree to share what they learn about the unknown attacker, later understood to be the Borg, and the Romulans depart with the warning that they are back.