As far as I am aware, the manga will go through a rather substantial shift in tone throughout its 4th volume (or shortly after that?). The adaptation pretty much went for the atmosphere the story will have (and keep) after that and changed the plot.. and some of the characters (>_>) because of it. I.. don't understand some of the decisions the team made tho and I also prefer a 'light start' over head-through-the-wall drama from the get-go. Altering personalities is a big "no"~ ^^°