The brush was, in a word, convenient. It hid his shame. For he was no longer a soldier. He was a deserter. The great Wizarding Wars brought their toll on everyone, especially the soldiers. It would be the soldiers who were forced to clean up after the wizards who fought their battles from their towers, hurling great balls of fire or ice or plague or whatever madness such monsters masquerading as humans could devise. And when those deadly spells reached their target, two things would happen. The target would either be too slow, and the spell would kill him and obliterate the tower, or the target would be too quick, and the spell would be blocked by him and obliterate the nearby village.
Either way, the spells left their mark on the nearby countryside. It... mutated the wildlife, forming monstrosities that, having been formed of violence, were violent themselves. Oh, they might not necessarily be carnivores, and actively prey on humans, but they might turn out like the hippopotamus or the cassowary- extremely violent, deadly, and territorial herbivores.
This led to the soldiers. They would be the ones who would clean up after the wizards. They would be sent in to kill the monsters the wizards forged of nature. They would be sent in to die.
This took its toll on a man, on a person, on a soldier. Watching your friend get gutted by a ten foot two jaguar, or a brother-in-arm get turned, by the ambient magics they were in for the good part of every day, into a conglomeration of snakes and devour his former friend. It took it toll.
Derran hid in the brush like a mewling coward. He waited patiently for the army patrol to walk past so he could run and escape. He knew of a town that lay in the north, where one of his brothers had moved to. His brother had said that there were no wizards nearby, that it was too cold for them. A wizard needed a certain level of luxury, and maintaining a temperate climate in the frozen north took energy. The patrol left, and so did Derran.
Derran Thurswrat is no longer a soldier in the modern times. His brother owned an inn, and Derran worked there with him, bartending and avoiding questions about his past. When his brother died of tuberculosis, Derran took over the inn. Eventually, he found love in the village, and married Angela Thurswrat. He took her last name for his own to obscure his identity in case the army were to come looking for him.
He is a man full of secrets, and refuses to give them up. The town villagers know that if they have a problem, they can talk it through with him, and he will never tell another soul about it. The secret would have to have grave consequences for him to reveal it, and some random stranger that walks into his bar (read, the PCs) would have to earn his trust and respect before he would think they were being honest if they told him that John was actually an evil cultist and they needed to know everything about him to stop him and his evil plot.
To earn his respect and trust, the PCs would have to do something admirable. Derran would not give them an idea of what to do, they would need to initiate it for themselves. Killing something would not do it, as Derran is a deserter; being brave and courageous in the face of grave danger is, to him, just the signs of another couple of suicidal idiots. No, they need to prove themselves dependable. Staying around town, helping people, proving that they are good people, this is how you earn his respect. As well as having something about them that he can connect to, or appreciate. This could be shared experience fighting the monstrosities that emerged during and after the Wizarding Wars, killing a wizard that was a part of the wars, or maybe deserting the army.
In a game, Derran could be used in two ways: a supplier of information, weapons, or help. Each of these things requires earning his trust. Information tends to be exclusive to the townspeople and their problems, but it could also be how the army works and operates, if the PCs need info on that and if the army hasn't changed much over the years. Weapons is his old army equipment. If the PCs need it, and earn his trust somehow, Derran could let them borrow it if they lack one of their own. And finally, he could join the PCs in a monster killing quest, if they can convince him to join and prove that the endeavor is not suicidal and has tangible benefits. Derran is not a coward, and is willing to endanger himself- he just couldn't handle the mental stresses of the monstrosities the Wizarding Wars brought.