Did i miss something ?

I don't know if i missed anything during the castellus tower. Zephiel is dead and I have seen people saying that to save zephiel by sacrificing Luze ( well , i never met an event with that choice till now?) I'm already near the end of the game and we're gonna fight the other Zeto, but Zephiel is still not back into the party. Is there something wrong? :(

There was a (kinda hidden) area at the Hollow Nest on the second map just after the cave where the chamber containing the hollow skills for the Hope Arc Team are. If you followed it up there, you would see a suspicious section of the wall with a different texture, and you'd end up inside yet another hidden lab (what is up with all these hidden labs in this game, seriously lol) containing Zephy's lifeless body. Talking to her would result in two decisions; one to let Capelle sacrifice herself to save Zephy (which doesn't happen either way), and the important one for Luze to decide to sacrifice herself for Zephy. This is the choice that everyone was mentioning lol.

Of course, you'd have to do this before you fought Luphiel rip as that area is no longer accessible after you fight him and the earthquakes.