Shinimori Aoshi
Shinimori Aoshi is a character from the anime Rurouni Kenshin. He was raised as a ninja/martial artist, who then worked for the Shogunate government. He eventually left his position there, and a few years later saw him working for a corrupt businessman, as well as with some henchmen that could only be described as creepy.
He encountered Kenshin at one point, and after he was defeated, he swore to himself (and to his friends who died indirectly because of the encounter) that he would do anything he could to kill Kenshin. He did everything he could to reach his goal, even to the extent of joining the sadistic Shishio Makoto so as to be given a chance to defeat the former Hitokiri Battōsai. There are times during that period that he appears deranged, being so single-minded in his goal that he neglects everything else that’s going on around him. When the two finally face off, Kenshin wins the battle but refuses to give Aoshi the death he was looking for. He eventually goes to the good side, by fighting Shishio to buy time for Kenshin.
He is the quiet, brooding character of the series, rarely showing any kind of emotion, including happiness. While he does not look it, he has a soft spot for his friends, particularly his protege, Makimachi Misao.