Curate, connect, and discover
Just read @intrepid-fictioneer-7's analyis on the madness of Servants, and it's got me thinking about Fujimaru's mental state and some implications of the lore.
We do know that they were most definitely a regular human at the beginning of the story; however, lore and dialogue imply that this is no longer the case.
The event where Vritra debuts has her note that both Mash and you seem both human and not human at the same time. It makes sense for Mash, as she's a demi-Servant, but why Fujimaru?
And then there's a bit in Koyanskaya of Light's profile - "Koyanskaya had planned to incorporate the Foreign God into her eighth tail, and to use Chaldea's Master for the ninth." To become one of her tails, it has to be a "Monster not present in the history of humanity." I think that says it all, really.
Fujimaru has been in multiple situations very similar to the ones that gave rise to heroic legends, having faced down the absolute worst threats to Humanity several times over.
And not just that - the very nature of pruning Lostbelts means that Fujimaru has an incredible amount of blood on their hands; enough that Kali is intrigued.
They've gone through far more than any human should ever have to go through, and it's implied throughout the story and confirmed that by Ordeal Call that, even if they were to return home at the end of it all, they would never be able to return to a normal life ever again.
Hell, Ordeal Call II reveals that Fujimaru not only has the potential to become an Avenger, but the most powerful one of all.
What I'm getting at is that what if one of the reasons that Fujimaru is able to empathize and connect with Heroic Spirits to such a degree is because they think like one, having fought through many of the same circumstances and now afflicted by that same madness?
Sure, they may not be as broken as some of those that they have under their command; but, as some have said, it's likely at this point that Fujimaru is the quintessential human to an utterly inhuman degree.
The way I interpret that is that he does things that would seem normal for a human at a glance; but if one were to examine it, they'd realize that Fujimaru's ability to do that goes far, far beyond the limit of what any normal human should be capable of.
For example, Humanity may be known for the indomitable spirit in the face of overwhelming odds; but for Fujimaru, it's seems that it's ontologically impossible for them to give up no matter what they're facing when even the most determined of humans would have given up.
A human, no matter how righteous, may still fall into evil if the circumstances are extraneous enough - but we actually see Fujimaru put into such a circumstance - and refuse to do so.
They can't ever stop moving forward, because if they do, it means all the innumerable sacrifices that have been made during their journey would have been for nothing.
Sure, it may be more subtle, but the underlying madness that would make a heroic spirit's mind incomprehensible to a regular human is still there.
"Fujimaru Ritsuka" may still be there, but it's buried under the being that is "Humanity's Last Master."