Would have helped to know what series that reference story was in and what it was really about... and what series you may want to read for.
I assume Love Ronin iby AJT is by Andrew Joshua Talon, and it is Love Hina story...
If so,
I then had to look the story up, as I've never heard about it, and I have read other stories by that AJT.
Having looked up the description, I had to compare to what I know of Mr. Cynical, whom I have read a few of his NGE stories of, and know that they are not for everyone (I quite Liked a few, like Detective Ikari).
With all that, I can also suggest perhaps some stories by WFRose from the Ranma universe, whom have a more hard case Ranma in them
Most of the more hard case NGE ones with Shinji overdo it. or have not been continued. But a few come to mind... examples: 'Thousand Shinji' by Academia Nut with a Shinji with a Warhammer bent. Also, several story arcs by GamerJay in the NGE universe crossing it with a Shinji raised by the Predator Aliens. And 'Alpha and Omega' by Xed Alpha, arcs 1 and 2, where he ends up in the Mass Effect Universe and is taught to survive by Aria of all people...
There are not too many hard case Keitaro ones out there, at least not with Quite the feel of what yoo would seem to be looking for... 'Contract Labor' by Greydon Creed comes to mind (I quite liked it and actually serve as a beta on it). Not the same type of badd ass, but 'Keitaro The Hater Of Women' by Necromonger comes to mind. Perhaps 'The Merry Killers' by The Sage of Toads may work for you as well.