Few 2hus have any actual personality in the games, fairy. You talk to them for, like, ten seconds and then you fight them. Reisen wasn't any more interesting in Imperishable Night, but eventually became a playable character. IN and SA also served to give non-main characters more dialogue to flesh them out.
Eiren never got a chance for characterization, really. She's never been one of those 'partner'-type characters like in IN or SA, and she's never been in a fighting game like more recent characters.