This COULD be me being delusional and insane, but I feel like for each passing Echo/human pair we get introduced to, the Echo is further removed from the human world. Because in the 4gamer interview with the Strange Eden team they talk about how Rin is the most removed from the human world but Senri is obviously the most integrated into the human world despite being an Echo. And Luka feels kinda well integrated, doesn't he? But I could definitely see people argue that Oujin is more removed than Yao, so I could be off the mark with this one.
(Side note: I don't think Mayoru should count into this because one, he's not a part of a Echo/human pair. But also Mayoru feels like he's a part off and frequently walks between the two worlds. He's said to mingle with humans often but he also works for an underground organisation that was created by and for Echoes, soooo... he also kinda acted as a diplomat in a way in the main story).