Writing Advice: Character Arcs
Start at the end. The last scene of your book or your series, how do you want the character to turn out? Do you want them to be a strong, confident warrior? Then you need them to start out insecure and not great at fighting. Do you want them to have a happy healthy romance? So start them without one, or in a bad one. This gives your character the most room for growth.
Every character has an arc that interacts. So you don't need to plan out every detail of the side character's arcs, but giving even just a one sentence 'character x wants to be helpful' can help you with scenes, and drive how your POV character(s) react to them. If you ever get stuck, you can look at those arcs and see who might jump in to help out.
Internal Conflict drives External Conflict. So external conflict is what happens outside of your character. It's the explosions they didn't plan for, the comet falling to earth, the sorcerer that wants to take over the world, or the arranged marriage they want to avoid, etc. Internal conflict is problems your character makes for themselves, through insecurity, jealousy, loneliness, misunderstanding, assumption, etc. So when you have an external conflict where your POV character is about to get married off to someone they don't like, their internal conflict of insecurity might make them stick to that plan even while they fall for the romantic interest on the side - but an internal conflict of assumption might make them come up with 200 ways their life will be horrible and spur them to run away.
Characters that don't grow aren't that interesting. Now this isn't to say that you can't have a character that is the best at all things - because those characters can have plenty of internal growth along the lines of becoming humble, meeting the first obstacle they can't overcome and having a crisis of confidence, etc. But every character has to change to give a story depth and connect to a reader. If you're struggling in the middle of a story, it might be because you didn't set up enough room for growth. You can go back to the start and add it, or introduce a new problem for them to deal with to fix that.