Axel/Larxene was actually pretty popular before Axel/Roxas even came into being. It's still pretty popular as far as het pairings go in Kingdom Hearts and so is Axel/Marluxia/Larxene. Don't get me wrong, it's not as popular as other OT3's, but it's up there. Also, despite what goes on in Chain of Memories, people still SHIP Axel/Larxene and tend to pay more attention to what goes on in the manga. Like most fandom, people just do what they want to do. I do it too. Heh. Anyway, five headspace thingies for Axel/Demyx.
1. Despite lacking Hearts, they are the two most emotional members of the Organization. Because of this, the loss is more painful for the two of them than for anyone else. However, since they still lack Hearts the emotions they feel strongly tend to be the negative ones, but not always. It's not uncommon for them to "half" feel things. Such as, "I should be sobbing, but there's only some tears". The lack of a Heart is why Axel can only be possessive in regards to Demyx. As for Demyx, many of his songs turn out "wrong"(that is, a song comes out angry or sad when it should be a happy song) and he often complains that his music doesn't "sound right", even when others say it sounds great.
2. Axel often snoops into Demyx's room to steal his snack foods(beef jerky, chips, candy, etx). He also uses Demyx's large vanity mirror, reads his song lyrics, and "borrows" some books. Demyx, however, doesn't dare go into Axel's room uninvited.
3. Axel gets irritated at Demyx's continued belief that they DO have Hearts. Axel knows that Demyx knows better. Once he told Demyx that he knew why his music didn't "sound right". He never did it again. Hint: Getting hit by fast moving water is like being hit by concrete.
4. They have done some very unpleasant things together. Nothing Saix, Vexen, or Larxene level, but they have been known to attract people through their song/dance routine and then turn them into heartless/nobodies. This ended soon after Roxas joined the Organization. Demyx considered it "awful, but necessary". Axel just considered it "necessary".
5. They annoy each other and neither one of them have much room to talk when it comes to being annoying.