Some TMNT stuff really isn't for little kids.
First an enemy, Oyuki Mashimi became a close friend of April O'Neil and the Turtles. She appears in Teenage Mutant Ninja Turtles Adventures, published by Archie Comics. In issue #28: Midnight Sun, Part 1, the turtles are in an airplane flying to Hiroshima to rescue their friends Chu Hsi and Fu Sheng from someone called Chien Khan, who we later learn is a mutant dog (but we don't know that yet because he wears a mask). Fu Sheng is an alchemist of some sort and Chu was a fireman who Fu Sheng gave the ability to turn into a Warrior Dragon. Chien Khan kidnapped them to use the Warrior Dragon in order to bring a demon to Earth. That is where Oyuki came in.
She was sitting on the steps of a building, smoking a cigarette (the Turtles' airplane was flying over at the time) when she was captured by ninjas and brought to Chien Khan. She is a feisty girl and threatens to rip his lungs out if he touches her. He tells Fu Sheng that he will kill her if he refuses to invoke the Dragon. Oyuki tells Fu Sheng, "You don't have to help him just to save me... life stinks, anyway. This proves it." But he agrees to help Chien Khan, anyway, and the Dragon is brought to the fore, though under a spell that keeps him under Khan's control.
While the Warrior Dragon is on his way to destroy a nuclear power plant to provide the energy needed to raise the demon, Chien Khan reveals that part of the ritual involves sacrificing a strong, young life. He ties Oyuki to a table and is about to kill her. In the meantime, the demon pops up but is stuck in the nuclear cooling tower until the sacrifice is completed, the Warrior Dragon snaps out of his spell, and Izanagi and Izanami (Japanese gods) show up and send the demon back to the underworld. Fu Sheng gets loose and tackles Chien Khan, knocking his mask off and revealing his dog face. Khan knocks him out and continues his attempt to sacrifice Oyuki, but the Turtles show up and Michaelangelo's nunchaku knocks the knife out of his hand. Khan gets away, though.
Later, in a more peaceful moment, Splinter suggests that the Turtles separate and explore Hiroshima for a while. Oyuki and Mike go to fly Japanese kites and she seems to like that he is into poetry. They make up haikus:
Rickshaws of pizza
Green Friends and mad dogs
From there on out Mike and Oyuki seem to be quite good friends. They sit next to each other when they go to a sumo match When the Turtles, Ninjara, and Splinter leave by boat for Tibet, Leonardo waves to Oyuki and tells her goodbye, but Oyuki only says goodbye to Mike.
In the April O'Neil comics, Oyuki has moved in with April in New York City. We learn that Oyuki lived on the streets and that she had to steal to get by. April says the one rule is that Oyuki can't smoke in the apartment, but Oyuki does, anyway. April gets fired from the station and she goes freelance Oyuki becomes her camera operator. Oyuki flirts a little with a guy that turns out to be a member of the Dog Stars, a gang that is run by Chien Khan. In the meantime, April witnesses a battle between the Dog Stars and another gang, the Golden Triangles. April and Oyuki track the Triangles through the sewers, but Oyuki's cigarette gives away their presence and April gets caught while Oyuki runs off to get help—but the help turns out to be the Dog Stars. Chien Khan captures the women and offers them a last request. Oyuki's last request is a cigarette. But April smells gasoline and kicks the cigarette out of Oyuki's mouth, into the gas. The place goes up in flames and they get out just before the explosion hits. Chien Khan is dragged to the underworld by the Demon he had tried to bring to Earth before. Later, April, Oyuki, and Chu are in April's apartment and Oyuki says she is going to quit smoking. She throws her cigarettes away, saying, "In the last 24 hours, my cigs almost got us killed twice and nearly burned down half of Chinatown." She seems a little weirded out that April and Chu are getting mushy, though.
The Turtles (and everyone else) are in the hotel room and are all very concerned about Mike, who is very sad about being stuck in the darkness. The Turtles decide to go out and take down Animus and April says that Mike can come, but Leo says that he has to stay. "But not alone, Oyuki can stay with Mikey until we get back." Splinter agrees, and Mike, who seems very lost in the situation, says "I understand, Master, it's all right. Oyuki and I will just hang out here in the hotel... in the dark." You can tell that being blinded has just drained all the joy of life out of him. We don't find out what Mike and Oyuki did or said while alone in the hotel.
While flying back to the States, the Turtles' plane is shot down by some robots, and Mike is caught and held captive by the US Coast Guard. He is interrogated and tortured by a doctor while there. Leonardo, Donatello, and some friends rescue him.
April later goes on a show called "Inside Affair". The host, McIntyre, has no great love for the Turtles, and his sole intent is to "expose" them as vicious monsters. However, Oyuki sneaks back into the control room and distracts McIntire's assistant Squint and gets the video of the whole torture and rescue shown on national television. After the show the Turtles spirit April and Oyuki off.
In 1999, in the storyarc The Angel of Times Square, Oyuki is not only still living with April in New York, but has graduated from being April's camerawoman and personal assistant to being a full-fledged correspondent for Channel 6.