Board Thread:General Discussion/@comment-4504826-20160516034744/@comment-28444460-20171017220419

If you're talking about Undyne... her being too determined was the whole issue. That's how she transformed into Undyne the Undying. If she was MORE determined, it wouldn't matter, her body is still that of a monster, and doesn't have enough substance to handle higher concentrations of determination. Heck, if she had MORE determination, she'd likely be stronger, but wouldn't last as long.

If you're referring to Chara... considering they managed to come back a long while after their death, I don't think a lack of determination is their issue. They had a plan, but for that plan to work, they had to die. That is why their determination didn't save them, because they were determined to die, not determined to live.