There's a mix of opinions between happy, sad, or an open ending. I personally think it ended happily, ofc they face a lot of depressing shit before they can reach it. I definitely think it was the perfect ending as it stayed true to the novel. If it ended any other way I don't think I would've liked it as much lol.
Spoilers...
No, everyone knows where KDJ is by the end, the open ending part is exactly how well or "completed" he'll come back as, as you just see everyone's reactions but not a full description of how he is now. It's a pretty meta ending since it is the personification of one of the themes of the story of some stories becoming so strong that they live thanks to the imagination of the readers. And we as readers are supposed to keep this story living by imagining what happened at this open ending and beyond. In my opinion, it is very happy ending, very well thought, though you end up thirsty for more so you go straight to wattpad and AO3 to read fanfics hahahaha
SPOILERS
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Just finished the novel and wanted to know. In the end before the epilogue we find out the skill KDJ asked for from secretive plotter was HSY's avatar skill. Did he ask for that skill bc he knew something like what happened in the ending was going to happen?