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.