Sinmir is a permanent resident of The Bannered Mare, where he will sit around all day complaining about the security in Whiterun. Speaking to Sinmir will cause him to say things about the Whiterun Guard and Commander Caius such as "The security in Whiterun is terrible. Shameful is what it is", "The guards in this city are all lazy and undisciplined. Things need to change...", and "Commander Cauis walks around Dragonsreach like a preening rooster. I could do his job blindfolded...."
If the Stormcloaks conquer Whiterun, he will be appointed as the commander of Whiterun's guard in replacement of Commander Caius. If he is Due to a bug, he will never leave The Bannered Mare, although he will now greet the player with "I'm commander of the guard, now." and claim that "The Stormcloaks can keep this city safe better than those Legion lapdogs." If he's the commander of the guard during the Thieves Guild quest Dampened Spirits, he will taste Sabjorn's mead poisoned by Dovahkiin at the Honningbrew Meadery.