One Fish, Two Fish, Half Fish, Marine Biologist.

Study.

"Kacchan do you mind if I study your tail?" Izuku asked as he pulled out the notebook Katsuki brought with him like usual

"What do you mean? You looked at it already." The blonde asked

"Yeah, but I didn't take any notes. I won't touch unless you say I can." Izuku gave an encouraging smile to the other

Katsuki looked at the other for a moment then shrugged.
"Fine." He said scooting back to get fully out of the water

Izuku carefully analyzed the black scales that lay flat against each other reflecting under the yellow sun. Then he saw something strange. Katsuki had two nearly flat mounds pressed against each other a few inches below when his skin fades into the black scales. Izuku didn't know what it was, but he refrained from asking this time. It might just be how the scales sit there.

"Your scales are pretty. Some are lined with orange like your fin." He said gesturing to the blonde's tail

"My arms are like that too." Katsuki said lifting an arm towards the smaller

Izuku smiled as he looked over the arm and nodded. He resisted the urge to grab the merman's bicep.
"Yeah, some are even different sizes and they move so fluidly." He said as if in awe and wrote down a few more notes

Katsuki rolled his eyes.
"They're supposed to moron."

"I know. I was just thinking out loud." Izuku blushed slightly

"Alright Ed Sheeran." Katsuki smirked playfully

"You know who that is?" Izuku asked in surprise

Katsuki frowned.
"Obviously, you think I only listen to ocean sounds?" He narrowed his eyes

Izuku immediately shook his head.
"No! I just... how?" He asked

Maybe it was a stupid question, but Katsuki lives underwater. How did he listen to music if he's always beneath the water's surface?

Katsuki shrugged.
"Sometimes the other extras play music when they clean up or whatever."

"Oh that makes sense." Izuku nodded dumbly

"Idiot." Katsuki scoffed

"Sorry. I'm glad Kacchan gets to enjoy music though." Izuku said

"It ain't bad." Katsuki shrugged

Izuku smiled.
"Do you mind if I look at your ear fins?" He asked changing the subject, but noting to play music next time he chats with the blonde

"Go ahead." Katsuki said leaning his head towards the smaller as he dipped his tail back in the water

Izuku looked closely at it seeing all the small scales.
"Wow, amazing. Thank you for letting me look Kacchan." He smiled as he drew a small sketch of Katsuki's ear fin

"Whatever." Katsuki shrugged

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"Um Kacchan, can I ask you something?" Izuku questioned finally

He couldn't leave it alone now matter how hard he tried. His brain wanted to know something and until he knew the answer it would shut up about it. So he just decided to ask the blonde.

Katsuki rolled his eyes.
"You don't have to ask that every time we talk dumb Nerd." He said looking over at the other

Izuku rubbed the back of his neck as he glanced to the side.
"Well I think it might be personal. I'm not sure." He said with a confused look

He didn't really. There was little information about Merfolk biology well to the public at least. So Izuku wasn't sure if what he was going to ask was inappropriate or not. The reason why he's confused is because well the area in which made Izuku's brain come up with a question was logically a genital area.