I have to confess that I am biased... I think that these two are just perfect for each other and I can't imagine them not being together. However, at the same time I can understand why people might doubt that there is any likelihood of romantic feeling between them.

Heiji is introduced quite early on in the Conan manga series (volume 10) and he is a memorable character, despite his personality not being fully developed. He shows up again very soon afterwards in volume 12 - where he uncovers Conan's true identity - and from then on is established as Conan's friend rather than would-be rival. However, Kazuha is introduced relatively late, not making an appearance until volume 19, when Conan and co come to Osaka.

Kazuha's appearance is quite memorable, as she mistakenly accuses Ran of being 'Kudo', a strange Tokyo girl who is leading Heiji astray! She acts very much like a jealous girlfriend rather than a concerned best friend, but when Ran asks if Heiji and Kazuha are dating, both deny it. Kazuha even goes on to say that she is Heiji's "big sister" who always has to keep him out of trouble!

Despite Kazuha's words, I think it's fairly obvious from the start that she cares very deeply for Heiji. The fact that she has used pieces of the handcuffs that bound her and Heiji together for a day (when they were kids) in a talisman that she made for both of them indicates some deep feelings on her part. Incidents occurring during Kazuha's subsequent appearances seem to bear out this theory, especially when she is still jealous of Ran! ^_^

Heiji's feelings are much more obscure... He teases Kazuha alot (although she teases him right back) and argues with her in the manner of someone who knows her very well. However, he doesn't demonstrate any overly affectionate feelings. The first proper hint that Heiji may feel more than friendship for Kazuha is in volume 23 after Heiji is rescued from the sea. He returns to the ship wearing only his boxer shorts and a talisman given to him by Kazuha which says 'Don't forget me you idiot!' on it. Heiji has a very sweet smile on his face when he looks at it and gets very embarrassed when he notices Ran and Conan looking at him.

However, the clearest example that Heiji and Kazuha care deeply about one another is in volume 28. Heiji saves Kazuha from falling off a cliff, but he is holding on to a branch with one hand and her wrist with the other... and the branch starts to crack. Thinking that they would both fall and die if this continues, Kazuha stabs Heiji's hand so that he will let her go and live. But Heiji hangs on, saying 'Even if I die, I won't let go' and manages to save them both. Later on, he looks at Kazuha and thinks: "I promise you, I won't let anyone harm you when you're with me."

So if they do love each other, why haven't they said anything? Certainly they have much better opportunities to confess their feelings than Ran and Shinichi whom they obviously parallel. But both Heiji and Kazuha are very reluctant to say anything that would betray their feelings and seem almost content to remain as they are, bickering and teasing... comfortable in each others company.

And that may be the reason why nothing has been said. Right now they have a good relationship together without any hang-ups, but once they move to a relationship beyond friendship then there could be complications. Perhaps they can't help but think about what would happen to their friendship if something went wrong with a love relationship.

Yet, I don't think much would change between Heiji and Kazuha if they were boyfriend and girlfriend. They would still bicker and enjoy each others company, but there would be an extra element to their relationship which might even be better that 'just friends'. So, the sooner they realize that then the sooner they'll confess their feelings! However, I don't think that's going to happen until something happens to push them into it!