Robert O'Brien is Trump's fourth national security adviser. Four in three years -- so, you might have an excuse. As an aside, the name of the secretary of Homeland Security is?
See this article, Trump's national security adviser out of sight in coronavirus response. I'm sharing it here because it reminds me of how different people approach prominent positions. I would have to say Robert O'Brien is more than taking a low profile. In the military we might say he is in "hull defilade" hiding behind his desk maybe?
The issue I'd take with his style is that he is supposed to be consolidating all the intel coming in from the different intelligence agencies and providing a comprehensive and blended report to the president. Sometimes, the NSA has to knock some heads with senior people. He is either unwilling or afraid to challenge some of the bigger players.
He also might just be the "yes man" that President Trump wants in the position. No arguing with presidential positions or statements, and as they said during the last presidential campaign, "Letting Trump be Trump."
He comes away from the position at the end of this administration (if he makes it that long) with the title Former National Security Adviser -- maybe that is his goal. And, how much did it cost to put a sensitive compartmented information facility (SCIF) in his house? I want a SCIF in my house!