Right now, Nexus can be unlocked before Glacial Peaks. In Nexus, Tassarion appears a little bit, but he is officially introduced in Glacial Peaks. Therefore, like the title says, Nexus should only be unlocked after Glacial Peaks. Here is a related bug report form 2022.
“Hero”/Tassarion actually exist in multiverses? That kinda thing is cool these days.
Saves devs time trying to explain why? 
Some other kingdoms have their characters messed up as well, I dont remember which exactly, but I definitely had encountered non-existant yet characters when playing quests. This issue has longer trail than just nexus.