"Hey, I'll catch up with you guys later." One brunette from a group of friends on the hallway said. One of his friends looked at him. "Where are you going?" His friend asked. "To the library. I need to study Biology for our test later." He answered that made his friend frowned. "You're not going to eat?" He just shrugged. "I'll take a bite later." He smiled and rushed on his way to the library. 

Two close friends. Have a big difference with each other. But such thing didn't hinder them to be close with each other. 
Typical types of persons, that's their similarities. 

"Hey." Donghae opened his eyes slowly and realized he slept on his book while studying. "Donghae." He heard his name and looked at the caller. "Oh, hi..." He smiled sheepishly at his friend and yawned. "Thought you will be in this kind of state, knowing you. Here, I bought your favorite bread from the bakery nearby and a box of chocolate milk." Donghae widened his eyes a little bit and smiled sheepishly once more. "Thank you, Hyuk-ah. You're my savior!" He said childishly as Hyukjae sighed. "Tsk. Alright let me help you study this ty subject." Donghae just smiled as he drink his chocolate milk.

No they aren't best friends, they just understand each other. Whether the one is happy, the other one joins its happiness. Whether one is troubled, the other one offers help. Whether the one wants to be alone, the other one let him be. Maybe they could call themselves as best friends. But they just don't care - the thing is, they know there is someone to rely to without any labels. 

"Minnie-ah." Sungmin looked and smiled as he saw Donghae. "Hae-ah. You need something?" Donghae shook his head. "Have you seen Hyuk?" He asked. Sungmin shrugged. "I guess he won't be coming to school. Try to ask Yesung, maybe he knows." Donghae thanked him and wandered around to look for Yesung, thinking why Hyukjae didn't attend school. He texted him already but got no reply since then.

And maybe Yesung knows, since they are inseparable, that's what Donghae thinks. He could say that Yesung is much closer to Hyukjae than him. It is true though, especially in others' eyes