Well, before it was revealed that Xykon took her diary by force I did consider the possibility that she was an agent of the Snarl who was trying to make sure the other Gates get destroyed.
Angels are Good (any). The archons are the always Lawful ones.I doubt Serini gives a hoot what a Deva thinks. I have yet to see a position taken vis a vis her alignment that argues LG; that's where Deva's hang out, right? In LG land? (Or am I thinking D&D 5e versus 3.x cosmology?)