Not a lot is set in stone yet, since I haven't programmed anything yet, but for what I have envisioned, to access Daria/Darius full route, you will have to have high enough affection and may a few key choices during the playthrough.
Eirik is controversial because of who and what he is. Daria/Darius is not (aside from being Eirik's loyal supporter). But being a bastard will be a problem for the relationship depending on your characters plans.