When writing any type of novel, we create a cast of likable, quirky characters for the cast. While some characters are created just for the sake of moving the plot forward, others are assigned major roles in the plot. These big name cast members are known as primary characters and secondary characters. With those characters, extra care and attention are made to make them likable not just to the audience but to ourselves as well. Because of this it is easy to get attached to the main cast, as if they were our own children. It is also very easy to favor other characters over others, if the cast member in question is a combination of some of your favorite tropes in fiction. These characters can even act like someone you sincerely look up to in real life. The name for these favored characters are creator's pets, due to the extra attention the creator gives them versus the other characters established for the plot.
While there is nothing wrong with loving some of your characters more than others, it can be dangerous to use this affection to give the character more screen time. When that happens, audiences tend to become annoyed over the extra attention the character may receive. They see an imbalance with the treatment of the cast, wanting the other characters to receive more time for character development or to have some importance in the plot. Worse, the favored character may get a free pass from the conflict and become unrealistically loved by all of the other characters. You may love the extra screen time your favorite character is receiving, but the audience will just become annoyed about it. In fact, your work could end up suffering for it as the potential the plot had for other characters is ruined by the imbalance in screen time. A creator's pet is a dangerous cliché that is similar to playing with fire itself. Both will potentially get you badly injured, one physically and the other mentally by the hate mail you could receive by fans.
Although it hurts, you have to treat all of your characters the same. You may love the character and want nothing bad to ever happen to them, but keeping the storyline consistent is more important in the long run. Think of it this way. Would you like only to have Firestar as the main focus in the Warriors book series instead of other complex characters such as Bramblestar and Jayfeather? Would you want story-lines only involving Light Yagami in the manga Death Note instead of the focus equalling out to L and his successors? Would you want only Eren to be the focus of the manga Attack on Titan instead of also Mikasa, Armin, and every other human trying to survive the Titan threat? If the answer to any of these literary examples is no, you understand why a creator's pet is such a damaging part of a story.
Balance is one of the most important factors to consider when writing any type of novel. It keeps your storyline consistent with strong character development on the major contributors of the plot. Without it, the story takes a nose drive into randomness and imprisonment in deep, dark plot holes. Characters are a huge part of this balance. Giving one character too much focus can interrupt this balance completely. Seeing your story as a scale that must be balanced completely half and half is a good way to see that consistency is stable.