Should I Choose Theodoros or Mestor?

Choosing Mestor causes Theodoros to lose his grudge against you. Mestor also happens to be the correct choice either way; he is the one who killed Herodotos's parents and potentially ruined the island of Samos for a generation. No matter who you choose in Followers of Truth, the story plays out more or less the same.

Which brother is it Theodoros or Mestor?

Mestor was the childhood friend of the historian Herodotos and his younger brother Theodoros who lived during the 5th century BCE.

Who brought the followers of Ares to Samos?

Kassandra: It's Theodoros. He's working for the Persians and brought the Followers of Ares here.