Tieria isn't actually dead, when Ribbons shot Tieria several times in episode 24 of season 2 , his consciousness was transferred to Veda courtesy of Regene Regetta, who had merged with Veda when TRANS-AM BURST was activated. Ribbons shot him, and then his body died. He transfered his conciousness into the temporary buffer data zone of Veda, where Ribbons did not detect him. Regene gave him the code that enables him to take over Veda but it needs Ribbons not to pay attention. Reference: Gundam 00 S2 Official File 5