The scene where Eragon visited Murtagh in his cell in the 1st book perfectly shows how close they were
Specifically, when Murtagh was talking about Nasuada and how beautiful she was. Maybe he didn't even realize how his words might sound, because he trusted Eragon and he could be himself with Eragon. He didn't even think that Eragon might judge him or use his feelings for Nasuada against him. And Murtagh wouldn't just bluntly say sth so personal, he grew up in the court so he knows that anything he says could be used against him
Murtagh was just himself, he didn't wear a mask and he trusted Eragon enough to speak freely around him















