Magos Kotov

Archmagos Lexell Kotov is an Explorator who lost forge worlds to Tyranids, Dark Mechanicus, and his own excavation of the Speranza, then took what was left into the Halo Scar looking for Vettius Telok and the Breath of the Gods. The crusade was a last throw. The throw hit.

Telok had gone mad and wanted Mars and Terra rewritten. Kotov fought him in the flesh and in the Speranza's noosphere with Eldar help he would rather not advertise, and the ark's own machine-spirit chose the Magos over the pretender. He came back poorer in worlds and richer in a ship that thinks.

Mars still counts him as Mechanicus, which is not the same as trusting him. Kotov has the Speranza. For a man who keeps surviving the loss of everything else, that is the remaining argument.