Headcannon that Daniela and Alberto's dad went to the surface together, and Massimo killed Alberto's dad.
So, after his parents threaten to send Luca away and he runs to Portorosso with Alberto, we see that Daniela and Lorenzo already know how to walk and blend in with humans.
While Lorenzo clearly hasn't seen his human form before (by his comment of how they looked horrifying after observing himself), Daniela just looks at her hair in disgust. She quickly gets up and starts to look for Luca while Lorenzo just follows her lead.
Lorenzo knows how to walk because he's just copying Daniela and gets assistance when they're walking out of the water. He obviously doesn't know what he's doing and is just doing what his wife does.
I also believe it was Daniela's idea to steal clothes from the laundry line, because how else would they blend in? Note that Luca didn't know to wear clothes until after Alberto gave some of his to him. (We'll get to that later)
Daniela also knows what letters are, and seeing as they live underwater, writing letters is impossible. Therefore, she must have been on the surface for a long time before.
When Daniela is describing humans from her time as child, she says they were just "sweaty land monster with a paddle." She wouldn't have known that unless she was up close with one.
The reason why she would be so adamant about keeping anything about the surface away from Luca is because of what happened to her friend, Alberto's dad.
Alberto of course says that his dad told him he left because he was old enough to be on his own, but what if he was just preparing for when the worst case scenario happens? What if he wanted Alberto to be prepared to live on his own in case of his untimely death?
It's never explicitly said when Alberto was abandoned, we just know he was gone for a long time. This could have happened when Luca was very young so he never knew his mom went to the surface before.
Alberto has had a keen interest in human things and their culture. We can assume he got that from his dad, who told him about life on the surface. That's where Alberto got many of his funny phrases and sense of clothing from.
Alberto never got closure of what happened to his dad because he was killed. Alberto thinks he just abandoned him, especially after being told he was independent enough. Daniela never came to Alberto because she was too afraid to return to the surface.
Massimo is known as the town sea monster slayer. He has several pictures, the harpoons, confidence, and the respect from other sailors to prove it. Not to mention the fact the whole town believes in them.
So what if Massimo killed Alberto's father? You see in the picture Giulia shows the boy a sea monster that sort of resembles Alberto.
It would be a viscous circle that Massimo is Alberto's adoptive father after he killed his biological one so I'm going to keep this headcannon.
There's probably many holes in this, but it's like 1 in the morning and cognitive function is a struggle right now lol
I just rewatched Luca with my brother and those were some things about Daniela I noticed so I threw this together because why not?
