Aaw, thank you so much! That's all super sweet and just wonderful to hear. I'll make this comment quick for now so you don't have to wait for a walkthrough, haha. Lu is by far the hardest to save, but there is a way to do it as long as you get on the right story branch and don't ever stop until it's truly over.
To get on the needed storyline for Lu you'll naturally have to romance him. Stay with him as much as possible and always say the things that make him feel better. You have to let Margret or Bemelle die, but you can't let both of them die. Then when you're on the shore at the start of the second night, the romance tip says you 'can' help the guide, but you don't 'have' to like it says in the Margret tips. Instead of helping or not helping, you can do nothing by letting the time run out. That will open up a different storyline. There are new ways for Lu to die, but one way to finally save his skin. Good luck! If you need more advice, just let me know :D.