Kenshin is just about to take on an opponent who is nearly as strong as he is, and a whole lot younger - Shinomori Aoshi, Commander of the Oniwaban group. After having fought past all of Aoshi's team to rescue Megumi, he knows this fight is going to be tough.

This expression is, to me, the essense of the true, adult Kenshin. Not a killer, not a goofball, but a man devoted to his friends, family, and country who isn't afraid to use his impressive strength to protect them. He's grounded, confident in his own abilities, and serious.