Both main characters are struggling to find their real identity. They have to deal with the unwanted power that is given to them and have to live through it as they unravel the secrets and mysteries of the environment that they're currently living. Uncertainty , confusion, and acceptance.
An introverted student is unwillingly given supernatural powers by a woman which makes them have to feed on other humans. The struggle of protagonists trying to find their place in both worlds is shown. Both share the horror/psychological genre and have a similar artstyle of detailed, yet crude.
Both protagonists go through a sudden change leaving them having to cope with not being human.
Another character goes through the same change, and takes a darker turn in coping.
Side character that knows both the characters and tries to help, whether they know in detail what happened or not.
Both the main characters undergo huge changes in their lives, and having to deal with it, whilst at the same time having to live with the knowledge that they are 'monsters'. The main characters are slightly alike.
These manga's were both made by the same mangaka so the art style is very similar... but i noticed both manga's had a similar atmosphere, the clash between the psychological and social norm is expressed in both of these manga's. The only difference is aku no hana doesn't really contain supernatural elements.
two amazing mangas with a gothic and phantasmagorical vibe that involve the beauty and horror of being young, in love, and a vampire (also, both artists are two of the very best in the business by a long shot) - read 'em both at yr. earliest opportunity if you dig a good tale of the living dead!
From both series, there is the theme of bullying as the main protagonist is ostracized by their peers. However, their life begins to take a darker turn after certain events. This includes taking a path that changes their life forever as they take certain measures. Both series is known for their violence as well as mature context. Happiness also has supernatural elements that changes the main protagonist drastically.
If characters being vampires / lusting for blood presented as a drama with a subdued sexualised edge is your cup of tea, read Lament of the Lamb whilst waiting for Happiness. LotL has more of an emphasis on family, with a strong incestous edge.
The mood, with a dark undercurrent and sexual tension, that both authors are able to create without doing anything outlandish is very similar and perhaps unique to them.