On the outside, character development is nothing but a change in their behavior.
At the beginning of a story a character will behave in some way and then change their behavior and behave differently. For example, one character may behave friendly towards the protagonist at first, and then turn unfriendly and work against them.
You could describe merely this change in behavior, but to the reader this will likely feel random and unsatisfying. Therefore, usually, you would explain the motivation behind the behavior as it appears at the beginning and the cause for the eventual change. For example, does the character behave friendly at first because they love the protagonist or because they hope to gain something from it? Do they turn unfriendly because they are disappointed by the protagonist or because they realized they could more easily achieve their goal that way?
Showing the motivations and internal changes of a character are how you interest the reader in that character. If you stay on the outside, the character and their changes will not be more interesting to the reader than a car that breaks down (a behavior change) and fails the protagonist.
So if you want your character to remain mysterious, you need another way to interest the reader in them. You can find good examples in how this is done in detective fiction. There, usually, we do not know who the antagonist is and we therefore don't know why they behave as they do. And yet we are invested in understanding their behavior (including possible changes), because they pose a riddle to us. The writer achieves this by giving us a character who has a motivation to understand the antagonist. In the case of detective fiction, the motivation is usually to solve the case and to convict the perpretrator.
Another example is alien encounter science fiction. In some stories the human explorers are faced with an alien intelligence that behaves in a way that they don't understand. The motivation then is the scientific interest to understand this alien species and why they behave as they do. In both cases, detective fiction and alien encounter stories, the reader interest in the character development of the mysterious other comes through the motivations and goals of the protagonist: to understand the mystery.
If the mystery is merely an obstacle (like a broken car) that the protagonist needs to overcome, but they have no incentive to understand it (the thirsty traveler will not care why his car broke down in the desert, they will leave it behind and search for water), then that character (the car) will not interest the reader either.