Yukio will try some youngdrink and instantly regret it and hate everything. But in the tunnels he'll be doing what he can to organize people and be a Responsible Person, plus he's like... faintly glowing. Not a great nightlight unless he gets some of his glowstick blood all over things, but people will be able to see him well so that counts for something?

AND THEN

tiny 6 year old yukio is extremely earnest and kind of curious but will mostly be crying and hiding and missing his family please help him he's so scared of everything