Joy is an exception and she never has as big a screen presence as Fangmei, Wong, Xiuying, or Ren. Joy is barely even a character. Joy is hot chick who gets kidnapped, gets annoyed Ryo is aloof. zzz. Tom has more character development than Joy. Goro has more character development than Joy. Fuck, even Mai done. Joy is terrible.
I can't say I really agree with your position on Joy, so I'll put forward a counter-argument to try redeeming her character.
I always thought she had a cool dynamic of being a rich and well-known socialite, but being feared/respected by Hong Kong's underworld. When a pair of roughnecks like the Poison Brothers get shooed off by Joy without any arguments, you know she's got some kind of power over them. Joy's spectre looms pretty large over the opening parts of Shenmue II, and she's definitely Ryo's gateway to adjusting to life in Hong Kong.
Her character arc is pretty Ryo-centric (which is not that bad, after all, it's his story), but it's definitely there. She starts as a flirty character, intrigued by the stiff and serious Ryo, but by the time he's trying to meet Ren, Joy knows he's a man on a mission and cuts out the flirting. In Kowloon, she becomes a more serious character when it becomes obvious there's more at play than a small-time gang war between the Heavens and Yellow Heads, and is integral to helping Ryo reach his goals. After all, she's the one who determines Zhu Yuanda's location after losing him to Dou Niu and Yuan.
She's also got little intriguing story moments that are easy to miss, like establishing her connection with Guizhang and Master Chen, coming to collect any debt you've left at the Come Over Guest House, as well as being one of the few story NPCs you can find randomly around the game world to talk to and get hints from.
Yeah, she has the trope of being the "sexy lady infatuated with the hero", but that's subverted later on as she gets further embroiled in Ryo's revenge quest, and genuinely becomes his friend instead. Yeah, she also ends up the "damsel in distress", but that isn't her sole purpose in the plot, and is part of a trio of "in distress" characters, including Wong and Zhu Yuanda as Shenmue II's story reaches its climax.
While I think, say, Ren and Xiuying are better characters, I think you've sold Joy a bit short.