@Xtinew About Vengo and Vic they might be 80 or 1000 yrs older but their counterpart female lead is also supposed to be older like that but aimed to be adolescent in face.
And about Changge, I think you are forgetting her original conviction. That is revenge only at first. But so many things add to it. She protects her people and wants to guard her country against enemies. She is not just a lucky chap surrounded by people who wants to save her. She is hunted as a fugitive from her homeland and she has lost her family through the most trusted person in her life- her uncle. She has trust issues. She saved Shuozhu, setting aside her grudge she saved Li Shimin from Khan's ambush just to ensure people don't fall in the hands of malicious Khans. She saved Luo Yi who was a valiant loyal general close to her father. But he chose to sacrifice to save her having no option left. She holds the most complexities in her emotions that is not just usual dilemma of choosing love or people, she has way too many things on her mind to find some time and think about Sun. And she saved Sun from Sher and Khan thrice. It's not that she doesn't care about him. She has to save the people she wants to save, she doesn't leave them to fate and regret later. As for Sun and all the people involving in her mess, it's also their choice in spite of her warning them not to because she knows she's like a mine field. That's the reason she pushed away Leyan, and also warned Adou but he followed her and lastly she is cold to Sun for this exact reason but he, too can't help protecting her at all cost because he sincerely loves her. As for her to realize this feeling completely, she needs time for growth to think things through and be less impulsive with her actions which is the main point of her character growth arc. She has had the most number of her loved ones leave one by one right in front of her eyes our of all characters in this series. She has the most layered emotional complexity. Even though Leyan felt just once through her journey about this loss, it's still far from Changge. She protects Changge because she knows she loves her but pushed her away to protect her, as Changge was the one to always protect her since they were children. Even after being hunted like a dog, she chose to fight for people using whatever skills she has learned. How can you call her selfish, seeing her just from Sun's love interest perspective? She got rotten vegetables thrown at her by the people of Shuozhu who even didn't know she was the one who protected them but she didn't care at all. Also when she shot the arrow at Li Shimin to save him, she took the blame for herself as a part of Wolf division to save Sun and his Eagle division. She didn't care what happens to her. She is anything but selfish. Yes, she's stubborn and reckless. These two words sound right. She is like a Fire Phoenix who's willing to burn herself after flying but she also makes sure others aren't burnt with her.
She's not a simple puppet like Hao Du, pointlessly hunting her down. He is the one who hasn't had any substance to his character, just blindly following orders with no avail.