Characters. While the plot may be the driving force of the story, it is the characters who you as a writer, and also as a reader, must be able to get behind, sympathise with, enjoy the ride with.
So how do we do this? The characters we create can often be a reflection of ourselves, or others that we know, because creating someone out of nothing except our own will and imagination, and then crafting them into a fully fledged person, is difficult.
But those we create don't have to be fully formed when we begin, in fact it is better if they are not, the journeys they go on with you, perils and pitfalls you throw them into, people you make them interact with, will shape them ever further.
I think our characters can be like a block of ice, ready to be sculpted. As we create them, their names, their backstories, their motives, we chip away chunks of ice to reveal the figure within, but from there we must continue. As we write the story, our character going though the adventures we have planned for them, more and more ice is chipped away, revealing more of the figure beneath.
The biggest change to your character is the one that will shape who they are and how they relate to you, the readers and of course other characters in their stories, will they change their tone entirely? will they learn from it?
Personally I think having a balanced leading character is key, of course you want your hero to be supported by the readers, but you don't want to have a perfectly clear and heroic person, otherwise there is nothing relatable to the reader. They need a flaw, maybe many flaws, things that make them angry, things that make them want to give up. Maybe they don't cope well under pressure and need to make a speech, maybe they hate goats and the villain has taken up residence on a farm.
I think this is in part why people often like villains, they can often be more relatable and more rounded than their hero counterparts, which is something to be acutely aware of.
With each character flaw, like and dislike, interaction with other characters, relevant or not, more ice will be chipped away from the sculpture, revealing the well formed, well rounded character beneath, and once you have that fully formed character, changes can still be made as they go on further adventures with you. To them, you are their parent, and they of course want to please you.