Blather, rinse, repeat

Yes, every aspect of their personalities is completely made up, even their character items (the food they carry around), but it's all universally accepted somehow! It's really cool how in-depth they ended up being, too.

KAITO, the blue guy, is kind of retarded, but he's sweet. His item is ice cream because he's such a sweet guy, in fact. Everyone always beats him up and pushes him around because he can't say no.
MEIKO is emotionally/physically abusive and a total boozer, and her item is sake because she likes getting wasted. She's nice to the kids but she pushes the adults around for kicks. She's the mother figure of the little cyber-family.
Hatsune Miku, the most popular Vocaloid, is a super-peppy "dumb green" with the mental capacity of a 7-year-old. She's really nice to everyone and her item is a leek... Because it's green, I guess. O_O She looks up to the older Vocaloids as her older brothers and sisters.
Kagamine Rin and Len are twins, but they're polar opposites. Rin is really outgoing and she's basically bipolar, because she can be super nice in one second and beating the crap out of everyone in the next. Len, on the other hand, is super shy, and he takes after KAITO in that he can't say no to people. He's also got some serious self-esteem issues, probably because his sister always beats him.
Kamui Gakupo was probably intended to be SRS BSNS, but fandom made him more like KAITO. He's nice and sort of doofy, and he can be chilly to his age-group every so often, but he gets along really well with the children. Not so much with KAITO because they clash a lot and tend to bicker amongst themselves.
And Megurine Luka, the latest Vocaloid, is probably the meanest out of all of them. Like most of them, she's nice to the children, but she totally freezes out KAITO and Gakupo's attempts at friendship. She's also obsessed with honorifics and she requires everyone to be polite at all times.

TL;DR but there you go. A BRIEF HISTORY.