Don't worry, it's not missable. I only used that term because I wanted to reference a certain something without spoilers. You'll understand when you get there.
I will say what is missable, however - no spoilers:
After every major event, go back to the Deep Dark Woods and speak to the NPC there. You'll get new dialogue and an item. The dialogue is missable, but, fortunately, you can get the items elsewhere. It is a bit of a pain to walk back, though, and arguably not worth it. This doesn't have anything to do with the ending and doesn't influence the story, it's just a nice bonus to those who remember the promise from the beginning (ie, coming to visit). I think the most important item you can get from him is a ring that increases your attack speed by one-star. You can make it later, but it's nice early on.