Here's a theory of why I think Iris joined the demigods: She probably lost all hope when she found out Lucas died and so when Lucas was mentioned you can tell sje looked upset. So she joined the demigods, to later betray them and accomplish Lucas's goal: to end the demigods.
but the thing is what if she did that but half way she gave up because the thought that lucas is dead came to her and she doesn't think she can do it without him. idk ╮( ̄▽ ̄)╭
Here's a theory of why I think Iris joined the demigods:
She probably lost all hope when she found out Lucas died and so when Lucas was mentioned you can tell sje looked upset. So she joined the demigods, to later betray them and accomplish Lucas's goal: to end the demigods.
Seems about right.