Kokichi appears to be amused by and in some way fond of Shuichi, having stated that the other boy seems "decently not boring", which is a very rare and notable compliment coming from him. He regularly calls him his "beloved Saihara-chan" in the Japanese version, and occasionally calls him "mister detective" or "mister Ultimate Detective". He also once called Shuichi "Shumai", as a parody to Kaito's "Maki Roll" nickname for Maki. Kokichi enjoys his company, finding him one of the most interesting person in the group and praises him often. He has also more than once mentioned that he is "constantly thinking about" Shuichi, and claims that he is his favorite whom he loves the most. On his whiteboard, it's shown that Kokichi has a more positive, though slightly unclear, impression of Shuichi, considering him "trustworthy?". Considering his hatred of boredom, it seems Kokichi is intrigued by Shuichi's development. During the trials, he often does not reveal Shuichi's lies even though he notices them. He occasionally made fun of Shuichi's nervous personality, calling his hat "emo hat" and calling him and Tsumugi a "plain pair".
As of Rantaro, Kokichi has been seen calling him "My dearest Rantaro," (In the Japanese Original it was "My beloved Amami~Chan"). Kokichi cried when he saw Rantaro's dead body. Rantaro was annoyed with Kokichi (like everyone else.) and didn't really pay much attention to him. Later on when Kokichi died and Maki and Shuichi was looking through his room it seen that Rantaro's wax figure that Angie made was in his room upside down. He chose it over Kaede, Kirumi, and Ryoma's, indicating that he has more interest in Rantaro than them.
During Chapter 4, when everyone is inside Neo World Program, Kokichi claims he would cry if something happened to Shuichi with a worried expression. He attempts to separate Shuichi and Kaito, telling Shuichi that he should be his friend instead. Kokichi says he can help Shuichi save everyone, but Shuichi doesn't trust him and leaves without saying a word. Kokichi then mutters to himself about being dumped and rejected by Shuichi, but he says he won't give up that easily.
When Rantaro met Kaede Akamatsu and Shuichi, Shuichi seemed confused how Rantarou mentioned about everyone gathering and getting amnesia. He thinks he is some way mysterious and has some clues about his backstory. Later, Shuichi became quite hostile towards Rantaro later in Chapter 1 based on his suspicion as being the mastermind and forgetting his talent. In the love hotel event He wishes to protect Shuichi, so he wouldn't lose another person that is very important to him. He mentions the possibility of himself dying and while he claims he was joking, Shuichi doesn't believe him. Shuichi tells Rantaro he would rather hear his story even if it is a risk rather than leave Rantaro to deal with it alone. In the end of the event, Rantaro feels very glad, trusting Shuichi even more and waiting for the time to tell him his story.
This ship most likely sailed when Saiouma shippers saw Rantaro's Wax Fiurge in Kokichi's room and decided to put them all together. Ship wars also came between Saiouma and Oumami shippers and so people just put them together to avoid it. Oumamasai is also shipped by Amasai shippers along with Saiouma and Oumami shippers. Though, the three of them do have more popular ships, namely with Kaede Akamatsu or with Kaito. Oumamasai currently has 62+ works on AO3.
- Amasai refers to the ship just between Rantaro and Shuichi
- Amasaiibouma refers to the ship between the three and K1-B0
- Amasaioumatsu refers to the ship between the three and Kaede Akamatsu
- Amasaioumota refers to the ship between the three and Kaito Momota
- Oumami refers to the ship just between Kokichi and Rantaro
- Saiouma refers to the ship just between Kokichi and Shuichi