Because a lot of times a mysterious character lacks personality and they're making them mysterious to make up for it.

Just by a little bit through. If I compare well-developed characters to mysterious characters, I seem to lean over to the mysterious characters. Like I prefer Jadeite over Luna, Courtney over Helga or Elsa over Anna. Even through I do like those characters almost the same. I still seem to lean over the mysterious characters more then the well-developed. I just like making head cannons I guess.

^^They don't lack personality most of them have a lot of personality. Like for a example Courtney's most of her actions,(aren't explained, if we are lucky we get hints.) how she's like outside of the game because of her competitive nature not getting in the way, her parents, etc. We don't know much about her. But she dose have a lot of personality, You can see that. Elsa is also a mysterious character and she's selfless, insecure, playful, protective, Elegant, and artistic. But we don't know much about her either. And Aurora and Jadeite's personality are hinted at. I guess you can be right sometimes, But most characters that are mysterious do have a personality.

If you have to think really hard about a character's personality because the writer didn't make it very clear, which is their job as a writer, than it's very poor writing. It's hard to enjoy a character when you have to constantly think hard about them to know who they are, which also takes away from the enjoyment of the story because you're too busy thinking about who the hell the character is instead of being able to judge the movie and the character. A well-developed character is very excellent writing, a mysterious character where you don't know much about them and they don't plan on completely showing who they are, is lazy writing.

Oh yeah!

"An Awesome Character" WOW! That is REALLY specific! *obviously being sarcastic*

^^^Believe it or not it takes a lot of work to write these characters. Especially for the ones that had to be hinted. It takes a lot of work to put nods in there. So you have clues on what kind of characters they are. If you don't then those characters are just bland and boring. Seriously do you see me calling Olga a mysterious character? No, I don't count her as one she's just bland and boring period. if the writers don't put at least some clues into their character to give you a bigger picture on what kind of character they really are. Then they aren't mysterious characters. And personality wise doesn't only count. Like I told you Elsa and Courtney is what I call good mysterious character but they personality isn't really hard to see. Why I call them good mysterious characters is because the writers put clues so the fans can figure it out what kind of characters they are. Sure, Doesn't sound the hardest characters to make. But they aren't as easy as you think they are to write for.

A well developed character who is also mysterious is the perfect combination :)
And I disagree with the (paraphrasing here) part you say about a character that is mysterious where you have to think hard about their intentions and mindset is a poorly written character.
In my eyes it's the exact opposite. I adore a character where I get to try and work them out. A character whose personality is painted so obviously to the viewer feels like a bit of a slap in the face.

Not being a know it all, but I think you mean "decide" not "decided" ;)

Really, an awesome character? That-seems sort of out of place.

My three favorite animated movie heroines: Elsa, Tiana and Anastasia fit this to a "T" It's actually my two favorite attributes in characters.