Notes

so jack lived in one of the areas of colonial america where samhain was practiced. one autumn, the winter came early and froze the lake, but not completely. jack died on samhain, and rose exactly one year later, on samhain night when the moon was full. he kept his brown hair, though his complexion darkened and his eyes turned amber instead of blue (they glow orange when he's emotional or being exceptionally powerful). he became known as jack o'lantern, the spirit of halloween, and helped the holiday evolve.
there are two types of spirits: those that were once living sentient beings, and those that are ideas given life. jack is a mix of both, because he was a person but jack o'lanterns are more of an idea. as such, he still needs belief in his person to be seen completely, but belief in his core or holiday allows humans to see him as a shadowy figure. this means that he and pitch have a cordial relationship, especially during october. jack also has a few select believers scattered across the globe who he helped believe during halloween.
he has the power to shapeshift, but only in limited quantities (he has a separate form called jack the pumpkin king, which is a tall, spindly, skeletal figure with a detachable pumpkin head that has glowing eyes. this figure is the older, more vengeful, terrifying spirit of samhain as it originally started. his second form is his usual form, jack o'lantern, which is the boy who represents commercial halloween, has a sweet tooth, loves pranks, etc).



so since jack is a child, he needs to have belief in a spirit to see it, even as a spirit himself. this is a problem because jack was born in the 1690s and the easter bunny didn't become a thing in america until the 1700s. when he died, he didn't believe in the easter bunny. he didn't know about tooth, either, but she was relatively easy to believe in once she got popular and when he saw her brigade of mice and minifairies. sandy and north had always had a believer in him. throughout the course of the movie, though, jack has no idea that the easter bunny exists. he thinks that the fourth guardian is merely always absent when he's around. this poses multiple problems (the least of which is that, while jack believes in the power of memories, dreams, and wonder, he has absolutely no hope (or he's inadvertently smothering the little nugget he has inside of him))
bonus for jack becoming bunny's last believer and preventing him from fading away.