Mei-Nyan is amazing, here's why: She's a fabulous dark foil for Soo-Won.
I'll start by digging into her: Mei-Nyan is jealous, vindictive, entitled, and, under it all, clearly afraid. She believes she has a right to the dragons' power as a descendant of Hiryuu. Remember, Yon-Hi was upset to discover that Yona was Hiryuu reincarnated because it took meaning away from the suffering of herself and her family. Mei-Nyan has the same feelings and she wants to correct it. Not only that, she is angry, she wants to take *everything* she can away from Yona, hence the reference to the Thunder Beast (Hak's just a prize to her, same as the dragons).
She's also clearly scared of the Crimson Illness. She's watched two family members succumb. There's an edge of fear to the expressions Kusanagi drew, that's fed into her willingness to take this risk and try to hatch a plot with SW. She's not stupid. It really would have made sense for SW to share these feelings. But SW just...doesn't.
SW could have been jealous of Yona. After all, that was Yu-Hon's first reaction to finding out that Il would be king instead of him. Yona took SW's place as heir. SW could have been vindictive or entitled, but he waited for a decade and decided that it was in the best interests of the country to kill Il, as well as satisfying his own wish for revenge. But those feelings were not his motivations. And he could have spent his life dominated by fear of the illness coming for him. But nothing he's done has been motivated by fear.
Not convinced yet? There're even closer ways they mirror each other: Mei-Nyan wants to take everything away from Yona. That is *exactly* what SW did. The dragons are Yona's home, and Mei-Nyan wants to take them away just like SW once took away Yona's home. There's even a mirroring with the Thunder Beast business: remember waaaay back in like the teens, SW insisting to Hak, "but I want Hak!"?
It's a great reminder that SW really is a weirdo, and wonderfully so. He doesn't really fit into *any* of the tropes, doesn't act like most people. Sometimes that's terrible, but sometimes it's wonderful.
Last note about Mei-Nyan: she's also an awesome character because she's familiar to anyone who's read about royal mistresses. She's an intelligent and ambitious woman, at a time when being born a) a woman b) not noble (presumably) severely limited her options. Her main tool to accessing power is clearly her body, hence the way she acts towards SW immediately and Joo-Doh and KS's determination to ward her off. At that time, becoming a mistress or a king's friend (hiya, Hak), a "royal favorite," was one of the best ways for common people to access real power. The restraint she still suffers from highlights the limits of that power.