idk what you mean by luka having no character. luka DOES have character, it's just that you choose it. examples: tons of random dialogue (levia, slyph, a ton of interactions between sonya, etc), insect queen side quest, first encounter of saja and raphaela, plansect village, magistea village, succubus village, choosing between alice and ilias.
basically what you seem upset about is that luka willingly accepts either the angel or monster world which are both shit, but he is also granted a third choice to go his own way, effectively denying both of those shitty options.
of course the third choice just leads to a dead end but that has nothing to do with his character, and that dead end could be extended in the full part 3 release.
also the reason why luka is nonchalant between interactions is for three reasons. A: because alice or ilias is already yapping and going crazy about it, B: luka is somewhat of a comedic relief, and C: luka is neutral on stances until you actually decide about it (it's almost as if you play as him).
I will agree that some people gained or lost character, but that is the nature of switching from a visual novel to an open world RPG with hundreds or thousands of interactions and the ability to recruit nearly every person you see.