My boyfriend and I recently read Aku no Hana together. He loved it so much that I'm hoping to find another similar series for us to share. He likes twisted stories like that with tangible character progression. He liked the romance factor in it, as well.

Demographic doesn't matter; shounen, shoujo, josei, and seinen are all acceptable. No hentai, however, please. Horror is great, but it can be any genre. Psychological is awesome, too. We really loved how intense it was. He thought it was hilarious how some plot twists made me sit back and cover my mouth as I tried to process them.

Any suggestions are welcome. Please and thank you!

Oyasumi Punpun has a similar tone at times; it's definitely twisted and with a interesting character progression.

You could also be interested in the author's other manga, such as Boku wa Mari no Naka and the recent Happiness (OSHIMI Shuzo). None are exactly like Aku no Hana, but the narrative style of the mangaka is very visible in these works.

Might just be me, but I got similar vibes in Aku no Hana from Onani Master Kurosawa. Had the same feeling of intensity I suppose, though for different reasons. The character growth is tremendous and there is a tad bit of romance from what I remember. Don't let the whole "masturbation" idea turn you off though (hah) because it really is a small part of the story and is necessary for character development.

maybe NHK ni Youkoso!