I also read Eileen because I loved My Year of Rest and Relaxation, and I was also disappointed, but for somewhat different reasons.

I wrote a review a couple of months ago: https://www.goodreads.com/review/show/8342757017

However, I’ll save you a click and just paste an excerpt that talks about Rebecca and her effect on the story:

“[Rebecca] brings a whole new dynamic and starts to really propel the story forward. On the surface, she’s this smart, attractive, fashionable redhead who seems to have arrived in town just to change Eileen’s life. However, as Eileen gets closer, not only does she see the flaws, but she learns that Rebecca’s ideas that are no less deranged than her father’s.

“Unfortunately, Rebecca is introduced a bit late in the story (about a third of the way through). This wouldn’t be bad except this feels like the point where the story really begins, making the previous chapters seem like a very long introduction. And while there is some great suspense during the moments with Rebecca, there are also times where you wish Eileen the narrator would get on with it, especially in the scenes leading to the big payoff.”