Luther had an idea that was a..."siblings bonding time" thing that humans did! right? so as he wanted to test something like this with you and Randal he had an idea which was...matching outfits, so he actually did it.

He had bought matching outfits for you, Randal and him! so you could spend "normal" sibling time like "normal" humans did!

So...here were the three of you in matching outfits Luther seemed more excited even though his expression was still stoic but he was showing excitement (internally he was)

While Randal didn't seem that excited (and not even smiling that crazy) then he looks at his outfit (one of the only times he's not using that copy of a gakuran) and gives a slightly irritated (and a little neutral too) look while looking at Luther and asking Randal: "really?"