According to a number of Grimoires and item descriptions, the guardians are great warriors who had died in ancient times. The traveller's final act, during its life, was to create the ghosts. These automatons would seek out great warriors and resurrect them in order to fight against the end of the earth and defend their home planet. The linked video, by Youtube user "MoreConsole" goes into more detail on the subject
What do you think?
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I think it's awesome that we're literally undead warriors from the past. It's makes you wonder who was your guardian in their past? [spoiler] My Human Titan was a Roman commander in his past. [/spoiler]