What is Cortana, the artificial intelligence that accompanies Master Chief through the Halo series? Is she a person or merely a thing? She speaks with the warmth and wit of a woman, and indeed her personality is based on the cloned brain of her creator, Dr Catherine Elizabeth Halsey. But she is also an AI construct, a hologram with no physical form or being, and with a life expectancy of just seven years. It’s a question the series returns to over its course, pondering whether the close relationship that Master Chief – and, by inextricable association, the player – develops with a computer program is authentic and reciprocated, or illusory and one-way; whether Cortana is your companion because of duty or because of love.