Don't Mess Up Your Character Introduction
Your characters only get one introduction, and from that very first moment, readers will form their opinions and impressions of that character. Once they have, it’s incredibly hard to change their minds, so you have to make sure that you make the right first impression.
In my last novel (currently querying right now!) I had a beta reader come back to me to say that he really didn’t like my female protagonist. However, in his notes on the actual manuscript, there were several parts he highlighted because he was very sympathetic to her—so why didn’t those parts sway him? It was because they came too little, too late. He’d already formed his thoughts on the protagonist as a person.
I heard recently that Finding Nemo’s flashback that the movie opens with was initially later in the film to explain why Marlin was such an overprotective parent, but test audiences really hated him, and by then, all the tragedy in the world wouldn’t convince them otherwise. Instead, Disney moved the flashback to the beginning so that we started the movie sympathetic to him, and were able to overlook or understand his more annoying/frustrating traits.
I’m saying that right out of the gate, if we’re meant to root for them, you need to show us why. If we’re meant to hate them, you also need to show us why. Don’t hold onto your cards, don’t keep secrets too late, because once we form an opinion on someone, it’s difficult to change.
When that opinion does need to change (such as when you’re pulling off a villain redemption arc), it typically takes the entire story and genuine character growth to do.
See my post here on why we root for a character.