looking for a shoujo

i didnt remember the ml and fl names but ml wears glasses and has black attire while fl is into lolita fashion with twintails.
also important fl has a knack of falling for red flags (like serial killers, stalkers,etc) and ml have to beat all of them, being her personal body guard (?) to find out about his sister's death.
there's one scene i vividly recall is the fl's brother swallow a whole bunch keys and trap them inside a room with him.
hope the description clear enough.