High Confessor Tektus has been a Child of Atom for over a decade, since the days of Confessor Cromwell. He eventually left Megaton with Confessor Martin to settle in Maine and eventually rose to the rank of Zealot. He was the one who helped save Grand Zealot Richter when he was trapped. After Martin left, Tektus became the High Confessor and he began to take a far more aggressive stance against the Harbormen.
He believes that the Island is Atom's holy land, and all its inhabitants must worship his glory. Though he is willing to maintain peaceful relations with the Acadians because they supplied the Children with the Nucleus, he is vehement in his hatred of the residents of Far Harbor, frequently deriding them as heathens and sometimes even sending some of his followers to sabotage their defenses.
Tektus can be convinced to leave the Island forever instead of being killed in the quest Reformation for a low speech check. He will then leave the game forever.
With a high charisma check, Tektus can be convinced to allow the player character to launch the Vessel's nuclear missiles, destroying the Nucleus with all the Children inside and thus allowing them to "reach Division".
Tektus is replaced with the synth Cole if convinced to leave the Island.
If you have replaced Tektus with a synth and then choose to disable the Far Harbor barrier, he will be upset as you have violated Atom's request for peace, however he will let you off with a warning, any future interactions with him will have him hope Atom doesn't punish you for disobeying him.