I saw some discussion upthread about Christine Chapel. I haven't seen the entire run of the original series in about two decades, but I seem to remember her being a major supporting character along with Sulu, Chekov and Uhura. Maybe I'm mistaken and she wasn't, but that's what I remember.
She was a bit more secondary. She made 25 appearances in TOS, versus 35 for Chekov, 51 for Sulu, 64 for Scott, 65 for Uhura, etc. Even if you throw in the 9 animated episodes she was in, she still comes in last among the recurring characters (well, second-last if you count Rand).
Still, I suppose it's the movies that solidified a lot of the public perceptions about the characters and their prominence. There's no reason the movies couldn't have given Chapel a significant ongoing role, but after TMP, they didn't. Really, once she became a doctor, having both her and McCoy would've been a bit redundant.
I agree - Chapel was a major recurring character but she was the most minor of the major characters, with the exception of her two guest star appearances in the first half of season one and a couple of other slightly meatier appearances such as the infamous episode with the inter-racial kiss. She was sort of demoted even further because of what happened later in the movie franchise.
The character suffered from being McCoy's assistant in sick bay, despite being a qualified biologist. The mistake was limiting her to a medical role instead of letting her utilise her established skills set i.e. McCoy and even Spock were portrayed as taking the lead in any bio-research episodes and Chapel was very passive, rarely taking any kind of lead herself.
In the reboot, they could have established her as an exobiologist, paeleobiologist, or bio-researcher but they stuck with the name-drop of 'Nurse Chapel' instead, limiting her options from the get-go, and then transferred her off the ship off camera! Most people don't care but I thought it was terribly disrespectful to Majel's memory, to the contribution of the character, and to the female dynamic overall, which was extremely male-dominated even before they wrote out one of the few recurring women.
I thought they were ignoring her in the comics because they did not want to reveal the likeness of the actress who would be playing her in the sequel but that proved to be totally wrong. I'm now hoping that they sent her off to the rim because they intend to feature the character in the comics in an episode where her path crosses the Enterprise's again but I may be hoping in vain once more!