I got the following sentence from manga 五等分の花嫁 (chapter 43 I believe), 俺はまだニ乃を理解しきれてないのかもしれない.
理解 is to understand and I'm thinking it's used with する here. It's verb stem is followed by きれてない. The closest thing I can think of is that 切れる is being used here but I'm not quite sure how that would make sense.
I think the sentence translates to something like. "I may still not understand Nino yet," but I could be wrong since I don't know what helper verb(?) this is.
Any and all help is appreciated. Thanks