The protagonist is mentally ill, he is super dull and can’t feel anything physically or emotionally. His half brother came to take carne of him so he can become normal again. At the beginning, he uses hypnotism to make the protagonist trust him. The story begin in the rain then hospital. Thank you
Those Years When We Killed the White Lotus vol 3 - I'm not sure if this is the one because they are not brothers but other than that it sounds the same