I am not sure how widespread the word "keener" is in modern North American slang. It was common at least in Canada, and I suspect it probably extends a little beyond that. It's common enough to be defined by Google (type define:keener), which is usually a fairly reliable measure of how commonplace certain words are or aren't. The OED doesn't have a definition of keener in the sense I present below, so it might either be fairly recent neologism or not be too widespread.
A keener is, well, someone who is keen to display her or his knowledge of a subject. A keener is that annoying kid who sits at the front of the class and always has to say, "ooh, I know, I know, ask me, ASK ME!!" whenever the lecturer or teacher poses a question, and very often even when no explicit questions are asked. Keeners are annoying and deserve whatever evil may befall upon them from their fellow classmates.
It is true that keeners may act in such a way out of insecurity. They feel a need to establish their superior knowledge publicly and loudly. This is the main distinction between a common geek (sometimes also a nerd) and a keener. While a geek (or nerd) may have quite in-depth knowledge of a subject, she or he usually feels no need to be a braggart and to constantly make a show of such knowledge. Real Geeks™ are smart and the buck stops there. Real Geeks™ are cool people.
In short, a keener is to a geek as an orc is to an elf in Tolkeinesque mythology: a deformed and unpleasant manifestation of an essentially benevolent and near holy being.