Ol' Buddy Ol' Pal

Ol' Buddy Ol' Pal is an achievement/trophy in Fallout: New Vegas which is awarded for successfully convincing one of the eight Fallout: New Vegas companions (ED-E, Craig Boone, Veronica Santangelo, Lily Bowen, Rex, Arcade Gannon, Raul Tejada, or Cass) to join you.

This appears to only be triggered by the very first companion you recruit so, if achievements are disabled at that time, recruiting other companions later will not unlock it.