Anything But Joyous
For an Eevee named Joy, she sure seems to have a lot of bite to her. Joy is best described as snappy and sassy, but only if provoked in that manner, and the only reason she behaves so is as a defense mechanism due to the treatment from her older sisters. Bonnie and Evan felt like they failed with the first three daughters, so they've given Joy more attention and the special treatment in hopes that she doesn't turn out "worthless" like her older sisters. This, of course, has made Joy's older sisters resentful, so they take it out on her, and make her feel unwanted, calling her things like "runt". Additionally, Joy is so much different than her sisters in terms of interests and personality, and it creates a lot of misunderstanding between them. They also annoy and tease Joy (usually playfully), use her as the test subject of so many unwanted makeovers, and their overly valley girl-like disposition just rubs Joy the wrong way.
This all sounds terrible right? Ah, that's sensationalism! Everything here is water under the bridge, and in the past!
In Poké Legends, where the Eevee Sisters begin getting more fleshed out, the girls finally come forward and deal with their feelings. When Joy is put into immediate danger, Rosie gives it her all to protect Joy at any cost. When they finally have the chance to come forward with their feelings, Joy's sisters tell Joy that they aren't really angry with her, but their parents, and that they wrongly took out their frustration on Joy who didn't deserve any of it. They let Joy know that they all really love her, and apologize for being insufferable to her. From that point on, Joy's bond with her sisters greatly improved, and when she decided to back Eevee in the all out war against Nyura, her sisters would be right there beside her.
... of course, that doesn't mean the playful teasing and makeovers came to a complete stop, even at the end of the series. Que sera, sera...
Outside of La'Vee Drama
When treated well, Joy lives up to her name, even though she may still utilize her trademark sarcastic sass every now and then. And, as a product of her very girly sisters, Joy knew how to play up her femininity when needed, usually against Eevee and the school boys, if there was something she wanted. But by and large, Joy is gender neutral and unconcerned with girlish appearances. She is respectful to elders and gets along well with the parents of the Poke Forest. She enjoys all types of play, especially if it's active, and seems to do pretty well in physical education.
Her close friend and mutual crush is Eevee, another Eevee who is just her age and the perfect playmate. She also feels like she is in love with him, though it took up until the end of the series for her to admit this, even at such a young age. You could call Eevee and Joy a Toy Ship, as they really have no idea what a real relationship is like, but that doesn't stop them from thinking they're in one by the series end! She is far more outward about her feelings about Eevee, but Eevee tends to only be open about them in private, making sure to keep up an image around other young boys even though he personally has no problem with girls (he even said himself that Joy's sisters are really pretty, which didn't earn him any favors from Joy).
Her kindred spirit is Chicolita, a young Chikorita girl who also has older sister problems. The two of them bonded almost immediately, sharing oh-so very familiar experiences. Joy and Chicolita would become very close and soon enough each other's best friend, confiding in each other all kinds of secrets about everything, sparing no details, no matter how gruesome. Joy was one of the first people to learn first hand about Chicolita's predictive nightmares, the ones that would terrify her so, and Joy was always there to comfort her about it. Being the very proactive sort, Joy would be one of the first people to go out of her way to do anything for Chicolita, and cared for her during a really awful fever far more than her sister Chikorita ever could have. By series ends, Joy and Chicolita have a sister-like bond with and love for one another.