Shantae's pretty (relatively) sullen in the original, at least based on my first impressions. Very little of that bubbly joy you see later on in the series.
It's a pretty incredible, believable arc concerning her self-esteem. At first she just plain feels inadequate due to "only" being a half-genie, and as a result has absolutely no room for shenanigans in her mind. After finally defeating Risky for the first time, her confidence improves somewhat, only to be shaken again upon being fired. Her half-genie status continues to weigh on her... until it comes to life and tries to kill her on Risky's orders.
Pirate's Curse is a strange case where even though she's technically even weaker than before and she knows it, in some ways it seems like a burden was temporarily lifted, no longer being expected to do genie things and thus, she can express herself more freely than before. In the end, though, she still needed to be half-genie to beat the Pirate Master.
Half-Genie Hero is where this issue is finally addressed head-on. After nearly having her genie half turn on her again due to dark magic, being saved from it by her non-magical friends and nearly seeing the entire genie race corrupted in the same way, Shantae is a completely changed woman by the end. No longer seeing the genies as infallible and fully realizing the strengths of being human result in her finally becoming at peace with who she is, and thus we see her at her happiest in the DLC campaigns.