Woman (Voice Recording):“All stations on the way will be bypassed. Please be careful.”
Shinji knits his brows.
Shinji:“I heard from my teacher that his job is important for protecting the human race.”
thewayneiac: He doesn't act like that impresses him too much.
Keisuke-kun: How does his teacher know this?
HeWhoPostsStuff: Maybe that's what Gendo told him to tell Shinji if he ever asked?
Hexon.Arq: I wonder if Shinji really thought that that story was total bullshit. The way he reacts here, he seems to be pondering that everything he was told is actually looking to be true.
OMF: The first mention of Shinji's mysterious "sensei". We never get much information about this person. We never see them. We don't know age or gender. We can surmise that they were Shinji's guardian, but beyond that we know almost nothing. You think such a person would have been discussed a little more. Perhaps Shinji wasn't very fond of him/her.
Additional Commentary
Kendrix: Note how Shinji speaks of Gendo: "Someone else told me big abstract things". On one hand, it goes to show that they're practically strangers, all he knows is what people told him, but on the other, he is regurgitating someone else's opinion here, and thereby hinding his own.
Under his resigned exterior, Shinji always had that unpredictable core of anger, his real opinion of Gendo is something less flattering than "He's a savior of our world", but here, he goes with what he should reply, not with what he actually wants to see. Until now, he's been freely chatting with Misato (as close as he gets to that), but now that an uncomfortable topic has been mentioned, he's building a wall. He averts his eyes to hide that nervous expression, he's breaking eye contact with her...
As for the sensei in question, we learn that he encouraged Shinji to try and play some musical instrument/apparently saw that he had some talent, so I don't think he neglected Shinji or anything. It's more that this person is a professional who gets paid to care for Shinji, so it would be improper if they did things that kids usually do with parental figures (Hugs! I don't think Shinji ever got any of those until episode 16) and Shinji couldn't really bond with that person as a family member... Their relationship was most likely not bad, but distanced, note how he always talks of his past life as solitary, but not "bad" in any way, not like there were arguments or anything like that.
Another important detail here (that often gets lost when adaptations and fanfics alike replace the sensei with some random relative/Uncle/whatever) is that Gendo put Shinji with a professional, whom he probably paid a lot of cash, his logic probably being that someone who was actually educated in how the minds of children work would get the job done better than he ever could. He didn't just abandon Shinji in a box, he actually took some steps to make sure he was provided for.
Of course, Shinji might've been better off with the random relative, as he'd have had some sense of family/adopted parents, as there are some needs that a mere caretaker cannot fulfill, but did these really exist?
Gendo doesn't look like he has any living family that should be trusted with a kid, and if the CI is to be trusted, Yui's family would be associated with Seele and twist Shinji's impressionable little mind to suit their needs.
That one snipped from ep 20 suggest that Gendo would like his son to have a normal life if not for the circumstances... I think he may have been trying to keep Shinji far, far away from the project.
As far as the original canon goes, I think that Gendo may very well have planned for Rei to be EVA 01's pilot (After all, Shinji's constantly told that she'll pilot it in if he won't) and for Shinji never to be involved at all, and may never have called him if Rei hadn't gotten injured. She was unlikely to win in her state, and he had no choice but to call in the one person that he knew would be compatible with Yui.
Consider this: He never makes any efforts to keep Shinji from leaving (Episode four!!!) even when it wouldn't be benefical to stay, nor did he, who played people like instruments left and right (Neutralizing Ritsuko with but a single sentence), tell Shinji what he wants to hear to keep him motivated
