I haven't paid much attention to those until recently, but since this event kinda requires a good one to get the most out of it (at least at my current level), I'm trying to use a specific one from my ally list. Thing is, I can't figure out how to get him to appear on the list of helper options.

From what I observed, the helper list is always the same if you fight the same boss (or a level I suppose) multiple times in a row. So if I get him once, I can keep using him until the boss is defeated. But once I summon a new boss, the list resets and he might not be there anymore. But if I wait an hour or two, the list seems to refresh again.

Is there any other way to force a refresh, or do anything to make it more consistent? Would removing all other friends help at all (pretty crude measure though, rather avoid that)?