It's all the Primarchs. Fairly simple. It's all 20- cough cough 18 Primarchs that you talk to. Even the dead ones, like Horus or Ferrus Manus, or even Sanguinius. Who are the Primarchs, you ask? Well...

The Primarchs are the "sons" of the Emperor of Mankind, created from Big E's geneseed to be the perfect humans. However, Big E treated them more as tools than his sons. Some father, huh?

"They shall be my sons, and in them will live the hopes of a unified Humanity. Theirs will be the strength to prevail, not only when victory lies within easy reach, but even when it seems unattainable, when doom settles like a shroud all about. In those times of darkness, my noble sons will shine the brightest of all." - Emperor of Mankind (assumed)