A battle ended, somewhere deep in the Makai. The demon the Spirit Detectives had faced was powerful, nearly an S class. The fight had been going on for a some time when Hiei called forth the black dragon, and while he didn't kill it, he hurt the demon badly enough that is was easy to finish off.
A fact that Hiei had never shared with the others, and never intended to, was that in addition to severe exhaustion that left him comatose for hours. Summoning the black dragon created the sort of high that was normally only attainable through the use of highly illegal herbage. As the dragons aura faded the enjoyable high seeped away, replaced by familiar bone-numbing exhaustion, he didn't bother to fight it. His knees buckled and he was unconscious before he hit the ground.
The detectives had a long way to walk before they would get to the portal. Since Kurama and Yuusuke both had minor injuries Kuwabara offered to carry Hiei. As the other two started walking toward the portal that would get them back to the ningenkai, Kuwabara leaned down, effortlessly scooped up the small fire demon and froze. Intellectually he knew the fire demon was small. It was the sort of detail that could easily be deduced at a glance. Kuwabara had devoted many enjoyable hours to teasing Hiei about that very feature.
But it was only then that it really hit him. Hiei weighed no more than a child. Shizuru could have carried him with little effort. He stared at the fire demon in his arms, and for the first time really looked at him. He saw his narrow shoulders, small chest, and the waist so thin he needed two belts. He had never realized how delicate the fire demon looked. He tried to squash the protective urge that swelled up inside him, dismissing it as lunacy. Hiei didn't need protecting, he had been forged in the fires of the Makai. Still he couldn't help but wonder if anyone had ever protected the little demon. His companions call jerked him from his musings.
"Hey Kuwabara, is something wrong? What's the hold up?"
"Nothing, I thought I sensed something, but it was nothing. Lets get home."
A little over an hour later, The detectives arrived at Kurama's house. Kurama Told Kuwabara to leave Hiei on the couch, to sleep. He and Yuusuke went into the kitchen to raid the fridge. Kuwabara paused by the couch, then still carrying Hiei he walked to the bathroom. A quick step on the scale confirmed his guess; Hiei weighed less than 90lbs.
"Someone will just have to make sure he eats more."
He walked back to the couch and gently laid the fire demon down on it, He then found a blanket and carefully draped it over him, before leaving to join Yuusuke and Kurama in their feeding frenzy.