Who do you think is the best villain that Togashi ever created in terms of character development?
Honorable mention to Toguro another great villain of Togashi. He betrayed his friends, believing that all of his effort in trainings to become a strong will all be gone to waste when they died so he decides to sell his body to turn himself into a demon w/c he totally despises or so to speak a punishment for failing to protect his students from a demon who literally kills them. A guy who thinks the only one who have a chance to beat him is Yusuke the successor of Genkai's power.
SHINOBU SENSUI (YUYU HAKUSHO) He was the former spirit detective who wants to get rid of demons, and to be the protector of human beings. He has been through many missions, and always had a stoic and ruthless demeanor when engaged in battle with demons. However, a mission that traumatized him, and cause a Paradigm shift in how he saw Light and Darkness between human beings and demons. He saw that humans were just as corrupt, if not more sinister than demons themselves. Because Sensui didn’t know how to respond to the given situation with this corrupt and bloody encounter, out of unconscious instinct, he eradicated everyone in the torture chamber for demons. This experience took a major impact in his mentality that he would eventually find himself dissociating his personality to the point where he would have multiple personality disorder. However, he was able to control those emotions, and his brain created them as a coping mechanism that handle that moment where all of his ideologies of humans and demons was changed forever. He developed 7 personalities, including his own that is considered to be the purest of all other forms. A great villain a great character IMO the best villain in shounen genre.
MERUEM (HUNTER X HUNTER) Let's be honest here guys at the start we all know Meruem as a prick and arrogant villain who claims to be the strongest of all he killed people w/ no reason and of course he is just another cell from DBZ, that chang when he met Komugi he started to se that humans are fascinating creatures, he started questioning their existence, actions, strength and power to where he comes to his own belief that the strong should protect the weak just by playing a game called gungi. And in the end he accepted his ant instincts, and merged them together with the human side knowing the pros and cons of both as seen with his views shared with Netero. Truly a great character, both plot, personality, and fighting wise. His development is the first one id seen in any shounen genre
as we all know Togashi is a genius when it comes to his characters (development, characterization) specifically the villains are out of this world.