Narrative designer Morgan Lockhart described the co-op nature of the campaign as the most difficult element of the game. Speaking about Cortana's fate in Halo 4, O'Connor stated that Halo 4's story was focused on the effect that Cortana's sacrifice and loss would have on the Master Chief. He also noted that this effect would be reflected in Halo 5: Guardians, with a story that explores how the Master Chief copes with loss and past memories.
Creative director Tim Longo explained that one of the main goals for Halo 5: Guardians was to expand the universe. To achieve this the studio created new main characters for Fireteam Osiris and introduced characters from the expanded fiction with Blue Team. Early in development, the team decided that Spartan Locke would be used to present the player with a different perspective on the Master Chief.
The aim was to create a deliberate contrast between the two main characters. In developing the characters of Fireteam Osiris, 343 Industries initially considered making the team Fireteam Majestic, using the same characters from the Halo 4 episodic series Spartan Ops. When Longo joined as creative director and the rest of the story developed, the developers decided that Majestic was the wrong team to use and set out to create a new fireteam with different characters.