KojiKen

Kojiken is the slash ship between Kojiro Hyuga and Ken Wakashimazu from the Captain Tsubasa fandom.

Canon
Both Kojiro and Ken debuted in Kid's Dream arc as players of the Meiwa FC, Hyuga as forward, captain, and ace, and Wakashimazu as the goalkeeper and vice-captain. Both are chosen and specially trained by coach Kozo Kira.

After the first National Junior Tournament, Kojiro gets a scholarship for the Toho academy, and Wakashimazu follows him to this new school in which they play the next two National Tournaments. This decision from Wakashimazu is put as a sign of admiration towards the other, and it's what coach Kitazume puts as the main reason for which Hyuga can't leave the Toho academy when he intends to, and it was as well used by coach Kira to try to convince Hyuga to go train with him chapters before.

In this Boy's Fight arc we can also see both of them worrying for each other in different contexts. First Ken, when Hyuga leaves to train with coach Kira to Okinawa, not only seems to be the only one along with Takeshi that knows where he has gone, but also shows a strong unwillingness to play the final of the tournament without Hyuga, being the one (in the manga version) to say to try to convince coach Kitazume and boycott the match if he still refused to let him play. When Hyuga returns he shows great joy, and even if Kojiro isn't part of the team, he continues to call him 'captain', to which one time Hyuga points out to him that he's not the captain anymore.

Later on, during the final, we can see Kojiro worrying about Ken's injuries and commanding the Toho to attack so he can rest without any ball getting to him, to what the goalkeeper shows grateful. These injuries had been the result of old ones caused by a car accident that had sent Ken to the hospital, there is a flashback where Hyuga is shown going to visit him there and bringing him something. There is a conversation between the two where Ken tells him not to bother to go, but Hyuga assures him it's no bother.

They also play together as part of Japan's ensemble.

Even if rivals and friends change rapidly in this series, the relationship between the two always remains as one of close friends. Moreover, even if Hyuga's attitude is usually aggressive, even towards his teammates if they do something wrong, this is never shown towards Wakashimazu, to whom, even when he takes a goal, he is always encouraging. He also relies much on him as a goalkeeper.

On Ken's side, we can see a great admiration towards his captain to the point he follows him to the Toho and considers his Tiger Shot the strongest. Whenever the goalkeeper expresses a trouble or wish, he usually approaches Hyuga instead of the team in general.

Their friendship is shown both on and off the football field, where they have great coordination, being, as Hyuga claims, the two best players on the team (Toho).

Fanon
Even if inside the fandom it at times competes with other ships that include Hyuga, it is still the most popular ship for both characters. This ship exists since the release of the first manga of the series in 1981, fanworks such as fanarts and doujinshis about the ship having been print published around that year, although many of them are difficult to find on the internet.

In fanarts they are often drawn along Takeshi Sawada, many times representing a family structure in which Takeshi would look like their son, having this its basis in the canon relationship between the three.

A thing to point out is that both's personalities are considerably different in the adaptation Captain Tsubasa: Road to 2002, especially Ken's, which changes the dynamic of the ship and is at times used humorously in fanworks.

Fans also point out a development in their relationship marked by the three scenes they have at the hospital, the first two in which Hyuga visits an injured Wakashimazu, and the third one in which Ken pays for the medical bills of Kojiro's mother.