Yeah I think it's a personal thing. Ella DEFINITELY has big plans for the future, but as the story has progressed, it's changed here and there.
We know she wanted to achieve Lvl 5 for the longest time, and she finally did so artificially. But upon obtaining it, she didn't receive a call presumably from Mother as a chosen one to carry out their will. If I'm not mistaken she mentions that it's up to us now, and that she's grown fond of us in particular.
We also know she has a list of peeps that she turns into superhumans based on potential and compatibility. So while the MC was NOT at the top of the list, nor the only one, she still liked that we were the one to pull through.
And then of course she's still collecting monster hearts and keeping a monster army and what have you do she STILL has a master plan, I just believe the 4th not selecting her as a sort of avatar made her plan less according, but not completely hindering it.
I guess what I'm trying to say is she really has no reason to be SOOO involved with the MC as people seem to think, and certain not enough to kill his Mom prior to ever meeting him in the first place, makes no sense. It makes more sense that she's just smitten with us to begin with, whether it be because we have the same monster parent, abilities, quirky lonerism, etc. We also go with the flow, as opposed to Jake's insecurity, Kenny's annoyment, Deryl's required hypnotism/obsession/monsterfication/jealousy, Cole's trying to fuck and rebelliousness, Charlie's brother obsession, etc. etc.
I'd much rather have control over a guy around my age, matching powers, easy to guide, friends with my friends, no crazy baggage, you get the gist.