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This depends on how the character sees himself (or if it's close third-person from someone else's POV, how that character sees him). If he is one sentinel of many — so it's a kind of descriptor, ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/33772 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/33772 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
This depends on how the character sees himself (or if it's close third-person from someone else's POV, how that character sees him). If he is one sentinel of many — so it's a kind of descriptor, like "the soldier" or "the doctor" — and he's somewhat anonymous, then keep it lowercase. If he is the _only one,_ then it's a proper designation: the Sentinel, the Winter Soldier, the Doctor.