Dan

Fuyutsuki is a good example of a character who was (presumably) conceived primarily so that Gendo would have someone he could talk to more or less as a peer, which would make specific kinds of dramatic scenes possible. But this functional aspect of the character inevitably implies certain things about Fuyutsuki. He has to prompt Gendo in conversation by pushing back against some of the things he says, which implies he isn't entirely aligned with Gendo philosophically. At the same time, he knows basically all the things Gendo knows and yet still dutifully serves him, which implies other things about Fuyutsuki's motivations. The show takes advantage of these functional implications to eventually craft a backstory for Fuyutsuki which explains some of these things about him. In an understated way, he's actually one of the more interesting characters in the series.