Ran Mouri and Shinichi Kudo

Ran Mouri and Shinichi Kudo are childhood friends and each other's canonical love interest. They have been friends and playmates since kindergarten.[1] Their friendship has received some censure from Ran’s mother, Eri Kisaki, when Shinichi would convince Ran to do things that could be considered dangerous, such as walking to and from their elementary school after dark[1]. They are very close to each other: even as students in their second year of high school, they address each other by first names without any honorific. Ran is occasionally annoyed by Shinichi's obsession with Holmes, and Shinichi is a bit disturbed by Ran's ability to crack concrete when she's unhappy,[2] but they generally get along well. Prior to the storyline beginning, both seem to have been clueless about each other's feelings. However, when Ran first meets Conan (whom she's unaware is really Shinichi), she confesses that despite his flaws, she really likes Shinichi — which causes him to blush and nearly confess his real identity despite his resolution to hide it.[2]
Conan currently lives with Ran and her father, Kogoro Mouri, who is a private detective. Ran treats Conan like a little brother, including often holding his hand or hugging him, once sleeping in the same bed, and once even dragging him into a hot spring bath together (the mere memory of it gives Conan a nosebleed and a healthy fear of Ran ever discovering his true identity).[3] Conan is very protective of Ran and jealous of her receiving any attention from other guys, going so far as to admit to Eisuke Hondou that he is Shinichi in order to forbid Eisuke from asking Ran out (and eventually to go to America with him).[4]
Non-canonically, Shinichi has confessed to Ran in the 4th movie 'Captured In Her Eyes', which she mistook as his attempt to get her memory back due to him unknowingly using the same words Kogoro used to propose to Eri. Canonically, Shinichi nearly confessed to her during the Desperate Revival arc, where it was implied that if he hadn't been interrupted by a murder case and then reverting to Conan, he would have proposed.[5] Additionally, during the visit to London, Shinichi finally revealed his feelings to Ran; while she didn't explicitly respond in kind, Shinichi figured out her feelings by the way she prompted his own confession.[6]
Shinichi continues to hide his identity from Ran because if the Black Organization discovers Conan's true identity, anyone connected to Conan or Shinichi would likely be killed to maintain the Organization's secrecy.[7] Ran has nearly figured out that Conan is Shinichi several times in the series, but each time Conan manages to create an alibi that she'll accept, since Ran seems to prefer that Conan not be Shinichi.[8] Ran would not be in danger just for knowing the truth, but as has been seen in the series Ran's behavior toward Conan changes the more suspicious she is about whether he is Shinichi.[9] If it were confirmed, one slip could be enough to tip off the organization, and Conan will keep up the charade rather than put her in danger that way, no matter how much he wants to tell her. However, he still remains in contact with her over the phone using the Voice-Changing Bowtie, despite the inherent dangers of staying in contact.[10]