These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fanon: There is a great deal of fanon about Takeshi and Akemi. Fanon has it that Takeshi had at least one older brother, and Akemi's only siblings were female. Takeshi thus took on the surname Kasuga (his wife's name), and this is why Akane and Kazuya's last names are Kasuga. However, the problem is that Kazuya and Akane's surnames have NEVER been given, either in the anime or manga. It's also quite possible that this bit of lore was created by usenet discussions back in the late 80s where it would be necessary to differentiate between the Akane the poster was talking about (Ranma 1/2's or KO Rs) hence leading to Akane being called Akane Kasuga. There is also an inherent contradiction, since if Akemi only had sisters, Akane and Kazuya would have a different last name anyway.