The Silver Dragon King is the second half of the Dragon God and has the Dragon God's abilities over the elements including fire, water, earth, wind, light, darkness, space, as well as possessing the Silver Dragon Spear.
When the Dragon God was defeated in the God realm due to causing too much chaos and conflict, the Dragon God split itself into the Golden Dragon King and Silver Dragon King after the Asura God cut him in half. The Silver Dragon King managed to escape the Divine Realm and remained in hibernation on Douluo Planet. She awakened when the Golden Dragon King was killed.
The author said on social platform that she didn't reincarnated as human just like Xiao Wu-spoiler from dd4 spoiler page.
Gu Yue had all the power of the elements while Na er had the power of the Silver Dragon Spear. Later in the Soul land 3, they combined to the Silver Dragon King's true human form, Gu Yuena, who is basically the appearance of Gu Yue and Na er combined. Gu Yue's hair colour was black, while Na er's hair colour was white. When they combined, Gu Yuena's hair colour was silver.
The Silver Dragon originally planned to kill Tang Wulin from the start to regain the power of the Golden Dragon King, but in a plot twist, she fell in love with him instead. At the end of the light novel, she chooses to die with him through his Golden Dragon Spear.
When she was revived in Soul Land 4, she was renamed as 'Nana' and became a Teacher. Whenever she meets her son Lan Xuanyu, she feels a maternal instinct but having all her memories lost, she still doesn't realize that he is her son.