((Does it seem like many Naruto crossovers star Naruto? I mean, sure he's the titular character, but there was a Eva fanfic starring Rock Lee that wasn't too bad.))
Kakashi knew Naruto Uzumaki was known as the Most Unpredictable Ninja. It was one of Naruto's actual nicknames in the Bingo Book, but before that, Naruto had a knack of performing feats that even he or Itachi would be boggled at. And now, he had taken one look at Naruto's latest feat before he turned to his blonde-headed student.
"See, Bushy-Brow, you do have a kid!"
His pink-haired student punched Naruto in the head. "You idiot! That girl's five years old. Lee would have been 12 years old at the time, and besides, have you ever seen Lee look at anyone else?"
"Hey, old enough to kill, old enough to..." Kakashi squeezed Naruto's lips with his fingers.
"Now now, not in front of the child." Kakashi looked at the girl. Sure enough, the kid had neatly trimmed long hair and big eyebrows, so he could see how Naruto could make that mistake. Also, the child was awake and looked at them. Oh, he saw that kind of glare in a mirror. "Alright, you're awake. Now, since my cute student didn't get your name, why don't you tell us?"
"...Satsuki Kiryuin." Rock Lee's eye's widened. Kakashi was the only one who saw this (as the kid normally has wide eyes) and pulled him outside the room.
"Lee, do you recognize that name?"
Rock Lee nodded. "Yosh. Before the Chunin Exam that the unyouthful Sound attacked, Gai-Sensei met the cool Lady Kiryuin and guided her around the village. Gai was very youthful when she stayed around, but after I got hospitalized by Gaara, she left. I think she wanted Gai-Sensei to leave with her, but Gai-Sensei loves the Leaf a lot. Gai wasn't youthful afterwards."
"I...see." Kakashi quickly snatched the reports from an orderly that happens to contain the girl's blood test. "Well, damn. It looks like Gai did sleep with her. He probably didn't know he had a little girl."
Satsuki had overheard the conversation. "I...then my father is..."
Kakashi did not want to tell the little girl, but Rock Lee beat him to it. "His ashes allowed the Will of Fire to become brighter. I'm sorry that you never knew your dad, but I would raise you like he raised me."
"Well, her mother could be alive..." Kakashi saw Satsuki hug Rock Lee tighter. "...Or we'll just fake her death and you can raise her. Ma, I can't believe I'm stoking the Flames of Youth."