Fanfic Servbot's review
If Kaoru's a Marty Stu, he's a very well handled one. Quite a few Evangelion fanfic communities had nominated or given him the Best New Character title in their online Evangelion Fanfiction Awards. A considerable feat, considering how Evangelion fanfic communities despise the Marty Stu.
Fanfic Higher Learning
I liked the fanfic a lot as a whole, but by far my favorite part involved a bit of Fridge Brilliance: I realized that Strike Fiss used BOTH the End of Evangelion and the infamous "Congratulations!" ending as part of his finale...and made it work.
Fanfic Willbyr's review
All in all, this was a lot of fun to read. There's plenty of WAFF but never enough that it becomes suffocating, the darker material is handled smartly, and Kaoru's presence never hurts the story or feels out of place, although there were aspects of the story involving him that weren't explained to my satisfaction. I especially liked how Shinji's experience of Instrumentality was handled. The omakes, for the most part, are very funny.
Fanfic EddieCurrent's review
This was my introduction to Eva. And it didn't ruin it for me. What makes it work, despite the happier take on the ending, is that Fiss tackles almost the exact same themes as the original, while bringing plenty more to the table.
Fanfic Probably the best example of a well done Marty Stu OC
This is a highly recommended classic Evangelion fanfic for a reason. The plot generally follows the events of canon, while going into more detail on the characters and what makes them tick, while demonstrating that their experiences were not only highly traumatic, but how having just one person to talk to and open up to can save them from a lot of pain.
The OC, Kaoru is an interesting case as it is hard to tell if he is a Marty Stu or not, largely due to his trying to stay in the background and not fix everything, despite it being implied that he possibly could have.
Instead, we have an OC who shapes and changes events by being an advisor and teacher, getting the cast to look at things differently and make decisions for themselves, causing them to grow as people and keep the worst things that happened to them from doing as much damage. The few times he does directly involve himself, it is not found out until much later that it was him.