I'm going to use manga Near, because he has actual emotions in the manga.

FIRST PERIOD

(Y/N) had no idea why she hadn't decided to tell the teacher she was bleeding from the lower parts. She thought maybe it was a scratch. Poor pure human.
(Y/N) also had no idea why when she started bleeding, the first person she thought of was Near. Sure, she was aware they were friends, but it was kind of weird that he was the first to come to mind.

Her only excuse was that he, the genius, most likely had some idea what was going on. Near was really smart, and Mello would just beat her ass, and Matt would...

We don't talk about that.

So, (Y/N) knocked on the door of the pure white room, and stood with the fakest smile she could conjure. She didn't want him to think she was too nervous. Then again, (Y/N) had a feeling Near would know she was nervous.

A creak came through the door as she saw Near's head poke out and look her up and down. He soon grew a small smile on his face as he opened the door open.

"Please don't knock down my card stack." He pointed to something that looked as if based off Big Ben.
(Y/N) nodded in response, "Now tell me, is there a reason you're here?"

"Uhm, yes. Don't tell anyone else Near, I don't want to picked on about this." She pouted, sitting down on

"And?"

"Do you know what it means if someone is bleeding from their lower half?" She rushed through her sentence, red faced and fiddling with her dress.

"What?" Near asked.

"I... I'm bleeding from my privates and I don't know what's going on..." (Y/N) muttered, looking away from Near.

Just then, she heard giggling come from Near. He had the normal expression of a laughing kid. She had always wondered why Near was thought of as weird. (Y/N) thought he was a kind person.

"Near! I'm serious!"

"Sorry... Mind me asking why you're asking me?" He asked, quickly gaining his composure.

"I... Well, don't want anyone else to know, and you're the most mature here." She whispered, relief in her voice.

"Alright, so you don't know what a period is?" Near asked, having some sort of innocence.

"Yeah, the thing that comes at the end of a sentence that's meant for a statement. Why?" Near facepalmed.

"No. Well, yes, but no. If you have a period, not in a sentence, but as a girl, you'll bleed from the lower half and usually experience pain. Most girls binge eat, get cranky, or bossy." Near explained, patting her on the back.

"Oh, well... Have I done any of that yet?" She asked, nervously talking.

Near genuinely had a grin on his face, "You usually don't complain, so I wouldn't worry about it. If you want to, I can ask Watari if we could go to the store for pads."

"Pads?"

"They're to cover up the blood." He said.

(Y/N) nodded, starting to wrap her arms around him, "Thank you!"

"Yeah, no problem." Near hugged her back.