Where is Merana, and when does she come around? I saw her once on the upper left of the map, but she's been gone now for days. Does she have a set schedule?

She comes randomly - she doesn't have a set schedule.

If you'd like an alert, you could subscribe to the hydrus pings: https://subeta.net/forums.php/groups/#/group/8173 Once someone sees she's on the map, they ping everyone else to alert them.

Thank you!