Susato and Ryunosuke's friendship is so compelling and solid because they trust and respect each other completely. It comes to an awkward juxtaposition with both of their relationships with Kazuma: Kazuma respects them and trusts them both, but not completely. Ryuu gets something: Kazuma will tell him there's something else he's supposed to do, but will not tell him what it is. Kazuma will tell Ryuu he has all the makings of a great lawyer, but will not tell him why he needs him to be one. Susato gets....not even that. She doea get something: Kazuma will make sure that she is included on the study tour, despite her not having any official qualifications or even being an university student. But she is not Kazuma's peer; she is his assistant, the little sister; she is not his friend. Not like Ryuu is.
Susato is Ryuu's friend and his peer. Susato might be his assistant, but functionally, at times, she is much closer to being his co-counsel, a lawyer with her own merit, solving the cases by his side. This comes to a head when she takes up the mantel of both the defence lawyer and being Naruhodo. Susato might be asking Kazuma for wisdom and courage for the task (standing in front of a grave where he is not in and never was) but when it comes to actually taking a page out of someone's book, it's Ryunosuke who she turns to. Meanwhile, Ryunosuke misses both Kazuma and Susato, but he in the text feels the empty place Susato left behind more. Perhaps it's because Kazuma is, at that point, dead to him (due to no fault of Kazuma's own) and missing him has become something unattainable; missing Kazuma won't bring him back. But Susato is still there, she is alive and she has carved her place next to him so strongly that Ryuu feels her absence like he is almost grieving her as well; he counts the days of how long they have been apart and feels depressed over how much time has passed. It's not the same without her, and living normally while she is away is a much bigger ask than living without Kazuma.
Then Kazuma comes back and it's not the same, and then Susato comes back and it is. Kazuma comes back and Susato and Ryunosuke are there waiting for him, on equal footing with each other, and Kazuma turns away from both of them just the same.
I don't want to drag Kazuma too much; he is young and still lost, despite how much more put together than Ryuu he seemed at the start. He is allowed to be that. The thing is, Kazuma doesn't allow himself to be lost; he can not allow himself to be lost. If he is lost, then he doesn't know where he is. If he is lost, then he has to trust someone else to show him the way. To be led by the hand out of the dark forest, instead of being the one leading; to be the one holding onto the hand extented to him, instead of being the one holding out his hand. And he is truly lost by the end, and threatening to lead everyone else around him deeper into the forest because he can't admit that he has no idea where he is going, and instead insists on the path he has taken, because he has been walking it for so long already and it has to lead somewhere, eventually; the forest can't go on forever. He just hasn't realised that even if the forest ends, it ends not because he will reach the clearing, but because there is ravine in front of him that he couldn't see in the dark.
Kazuma and Barok are very similar in this sense; both of them are inherently distrusting of the world around them because they have decided to be that way. Barok just stopped walking towards the darkest part of the path without a light slightly before Kazuma did because he recognised eventually that doing so would be utterly idiotic and self-destructive. Kazuma needed to be physically pulled back at the last second, but he did stop trying to break free eventually, too.
Ryunosuke isn't some paragon either; he isn't some flawless saviour. He simply decided to trust others, and let others come to trust him. It is much easier to find your way when you are walking with a group where one has a map, the other has a compass and the third has a torch; and even if they didn't have any of those, walking in the dark is less scary when you have a hand to hold onto. Ryunosuke and Susato found each other in the darkness and decided to hold onto each other, both of them trusting the other to lead the way when they couldn't find it anymore; neither of them was dragging the other somewhere they couldn't be sure was the right way. And here they are now, right where they both want to be.