Even from the maid’s flashback we don’t have much info about his brother besides he’s better than Zenon.
As for Yuno, Zenon did notice his personality is different in the game. He’s more goofy here; meanwhile when the MC watch the anime in marathon, he mentioned Yuno is just as unbearable as Zenon in different kind of way.
In this world, Nova noticed Zenon’s talent and fast forward the scenario almost 3 years earlier. And as result she also look for other characters to join the early admission.
So, something might had happened in the original story during that 3 years to change him. Or it’s just result from his adapting alone to the academy’s culture without a buddy. Yuno did say he’s bad in using formal language, but without Zenon to back him up, he would be alone and forced to adapt by the environment.
He might just imitate the haughty speech of other student, and limit talking to avoid making mistakes in the first place, which ends up making it seems like he’s looking down on people.
The usual harem MC’s convenient timing hearing problem is there from the start though.