if we put shu and the children he adopts like batman under the lens of the psychology of how children view their parents and based on my own family dynamic then ...i see it as boa, the first born, shu's first student, being the kid who gets the brunt of Shu's anger, gets an experience to what he's really like, and basically sees Shu during his worst as someone who is by no means perfect, which comes to fubuki, the second-born, shu's second and of course, favorite student, that he saw as a second chance as i often reiterate and gets to remedy the mistakes that shu did to boa which means that Fubuki gets a new version or rather the Shu that he was in the first season and unlike Boa, Fubuki sees Shu as someone who is perfect and can do no wrong or rather sees Shu as someone who's trying their best to make up their mistakes.
Now comes to Lain, the third-born, his last student as we see in the series, someone we see Shu coddles in his own way and is at his side and much more focused on since he's technically the baby (and he bears the same resemblance as Shu being power-hungry), Lain finds Shu overbearing, he finds Shu to be unpredictable as evident when Shu follows him, seemingly even heading the same path as him, only to realize that oh, you've been guiding me all along, this is what you wanted to show me. Lain wants to beat Shu and nothing like with Boa who wanted to beat Shu as Red Eye because he wanted him to see Boa but Lain feels as though he wants to usurp Shu's position of sorts, especially now that Lain is ranked as No.2 in the world, something Shu had been prior to the series.
Thirdborns are more often than not already independent and often feel like they know better will tend to try and outsmart their parents and openly challenge them so I see Lain and Shu's relationship as that. Boa and Shu's relationship is trial and error, Fubuki and Shu's is hero worship and Lain and Shu's relationship is something that of rivals.
