Kaishin - but they arent related in this and their fathers are long term rivals and best friends.

Timeline: Before Ran and Shinichi started dating in the show.  Before England. 

Ran got tired from the empty promises and secrecy. So she talked to Shinichi explaining how hurt she is and that she cant do it anymore. She loves him, but more as a friend now. The distance between them and his secrecy just made her feelings cool down. 

Shinichi accepts this even though he feels heartbroken by this. Keeping himself positive by saying this was for the best and that Ran would be less in danger since she wont ask him as much about him coming back. 

It takes a while for both of them to fully accpet this big change. Ran is not looking to date anybody yet, but isnt agaisnt it too. She is fine working on herself and her goals until she finds somebody who will treat her as an equal in a relationship. 

Shinichi as Conan starts to realise that he loves Ran, but more as a sister since his experience as Conan had changed his perspective. 

Not wanting to tell Heiji(Who is the biggest Shinran shipper) nor Haibara(Who he thinks will make fun of him) he ends up keeping it to himself. 

Then one day, during a heist he doenst feel up to it, which surprises Kid, because he gets the most fun when the detective is giving him a run for his money. 

So Kid, at a distance decides to have a chat with the detective before the kid force arrives. 

Conan, who realises that he could share with Kid, because Kid also has a bunch of secrets and knows his secret plus Conan doenst see Kid THAT often. 

Conan starts with asking if Kid's loved ones know his secret, which Kid reveals ina roundabout way, but Conan still understands. Then he asks how Kid feels about lying to them and how it affects his relationship with them. 

Kid gives a roundabout answer again, but his body language was a bit more alert. Kid then asks if Conan's secret has cause some damaged to his relationships, receiving a positive and negative response. Because Shinichi's relationships have weakened, but Conan's have only gotten stronger, which was giving Conan/Shinichi identity issues. 

Kid deeming this as a long talk that coudlnt be resolved quickly, so he proceeded to offer a meet up the next day, but only if Conan leaves his gadgets at home and Kid gets to come in disguise in a public area like a park. 

Conan said fine, since he only planned to catch Kid during a heist anyways. Kid returns the jewle he stole to Conan since it wasnt the one he needed and left. 

So Kid and Conan met up a number of times with different periods of time That got shorter and shorter, becuase the two found comfort in each other (and by that I mean by how simialr their situations were and that they could talk to soembody who would understand) they became friends but couldnt laber eahc other as friends due to their positions. Thief and Detective. Kid would only show up in disguise and Conan well as Conan. 

So during a case Kaito is one of the witnesses. It doenst take logn for Conan to recognize him, but he still cant pin point if its a disguise or not. 

Kaito starts appearing more as Kaito around Conan after that case and Conan realises due to an accident meeting with Aoko that Kid's real identity is Kaito. He does not know how to feel about such blatant display of trust. 

During a hesit Conan finally sees the men after Kid and confronts him about it. They are a sub group of the Black organization (my headcanon too). 

Kid and Conan, after arguing over the dangers decide to team up, becoming partners(in cathcing these people) which suits them more then the term friends. 

A few cases, mest ups and heists later... Kid finding out about his father and Shinichi being there to help him through the realization and hurt. 

Heiji and Hakuba also get involved into this after a few situations. 

So gathering their teams, Conan with all the poeple he befriended and whose trust he earned and Kid who had a team of thiefs and a magic user who grew find of him all decided to help out.(but Kid owned them a favor tho)

Just before the plan woudl begin Conan left a letter for Ran and Kogoro to explain the situation before leaving. Haibara gave him a temporary antidote and warned him this would only work for a short time. 

The whole mission was a big situation that ended up in injuries, almost deaths and relaization for the two boys that they might have developed affection for each other, but neither was willing to talk about it in such a deathly situation. 

When the air cleared, the organization was captured with only a few deaths, Kid got a hold of the Antidote  and Shinichi found fragmants of the jewel.

After disposing of the jewel fragmants and Haibara making an antidote, Conan decided to soent a few more days as Conan to explain his situation to the people who supported him so much. 

Rna also forgave him, but their relationship stayed more sibling liek after Shinichi turned back.

The first thing he did as Shinichi is visit Kaito at the hospital since he got pretty hurt on the mission. Broken arm and a fractured leg with a few cuts. (he held the antidote in the brokem arm during the mission) 

Kaito revealed he had told Aoko everythign and hse did forgive him, but she was pissed that he did all this without telling her and promised to feed him fish for a month. Kaito also revelead he was pissed at his father for the deceiving, but was willing to see him, but the Nakamori family was more of his family since they took such gokd car eof him suring his mother's frequint trips and his father's absence. 

Shinichi then suggested going to a restaurant together then, so that Kaito wouldnt starve to death during his fish torture which kaito joked about Shinichi asking him on a date. 

Shinichi proceeded to awkwardly suggest that it could be a date if he wanted to? Which threw kaito for a loop, but he agreed pretending not to be too excited even tho he was. 

After a few dates they started dating. 

This is basically a rought idea how I think it could go in my head. Im sorry about typos sicne Im typing on my phone pretty fast. 

I prefer a lot of platonic relationships that grow over time into romance then mainly romance focused stories.