To deduce the character of the sentry, let's turn to the sentry's words and actions for some clues.
First, a brief description of the sentry himself: he is eighteen years old and new to the job. In the story, he is proclaimed to be the "weakest spot in the defenses," presumably by the rebels themselves. He is approached by a girl of about fifteen (named Mariposa), who leads him on a wild-goose chase for her cat. The cat, we later learn, doesn't belong to the girl. It merely serves as a diversion, so that Mariposa's accomplice can replace the occupying forces' flag with one of their country's own.
Because the sentry himself is young and inexperienced, he is deceived by Mariposa's pitiful appearance. However, we must bear in mind that the sentry is also a tenderhearted young man. This can be evidenced in the way he treats Mariposa. He is reluctant to be unkind to her, awkwardly reassuring her that the cat will appear again in the morning.
As a nightwatchman, the sentry has orders to "treat the natives politely and considerately," as long as they aren't making trouble. For his part, the sentry wants to follow the orders he's been given, which demonstrates his loyalty to the occupying forces. However, he isn't exactly a battle-hardened warrior either; so, to rationalize his less-than-soldier-like behavior to himself, he concludes that Mariposa can't possibly be a threat, and that his kind actions actually fall under the jurisdiction of treating the "natives politely and considerately."
Later, when his major addresses him, the sentry defends his actions. He politely argues that Mariposa was actually looking for her cat. However, the older soldier quietly explains that the cat belongs to the caretaker of the grounds and that the rebels always send out "kids of fifteen." Because of his inexperience, the sentry finds it difficult to understand why a flag should be the cause of so much trouble; he doesn't realize that the rebel flag is an important statement of rebellion.
So, from the story, we can deduce that the sentry has not yet lost his innocence; he is by turns kindhearted and compassionate.