He's killing Boko Baras by the postbox, as Deku thought. 

"Hey, Sis." 

"Hi. Postman come by today?"

The postman has not come by today.

But that might be him in the distance. Their vision is still most of what it was before so he's at least five or ten minutes' flight away.

The twins have never really spoken to the Postman, so they decide to wait for him.

He lands near the post box. "Oh, are you two the new - not quite humans?"

"I'm not sure if we should still use the word, but me and my sister used to be called vampires."

"As good a word as any."

"We used to eat people." She says this casually, as though she's glad that's no longer the case, but she doesn't feel burdened by the fact it used to be.

"I see... You don't anymore. What have you done to atone?"

"Um, we gave up some of our vampire awesomeness. And apparently our great-great-great-great-grandkids are going to have even less. Which kinda sucks when you're immortal."

"I suppose that's something. I cannot imagine what my life would be like without Valoo's blessing."

"Is that the dragon guy?"

"Indeed so. He gave me my wings and lets me read the wind."

"That sounds so cool! We don't really know who made vampires, but whoever it was lets us turn into cats."

The twins proceed to do so.

"Interesting." He fishes through his messenger bag and produces a letter. "Do tell Hollo that there is mail for him when you next see him, please."

"We will."

"If you are ever inclined to visit the Rito Tribe, we live on Dragon Roost island. It's due north of here, perhaps twenty hours' sailing in a good wind."

"We could totally swim there."

"I could not necessarily recommend that. There are some fairly unpleasant creatures living in the sea."

"Oh?"

"Rocsquid come to mind. They shoot rocks at people. There are giant octopi here and there. Strange flying fish that like ramming things."

"Thanks for the warning. We're going to go find Hollo now, bye."

"Goodbye." He finishes inserting mail into the postbox and flies off to the west.

The twins search for Hollo's scent.

He's in his potions 'shop', a little nook in the thick wood walls of the Forest Haven about halfway up.