Her voice is lively and sharp, so I've always transferred that over to her personality. She's lively and loud and has strong opinions and a desire to make them known. She's determined to formulate her own opinions on things, and will not stand to be a pushover, or to see others being made one. A strong sense of justice and a willingness to look out for others. She has a kind heart, but when the situation calls for it, she's willing to be mean. She makes friends quickly and easily, and even those she isn't close to seem fond of her, even if just in a vague "ah yeah she's cool" type of way.
You put her next to her brother, and despite their similar appearances (I imagine the same height and skintone), they're such polar opposites that people can't believe they're related. Well, not by blood, but you get my point. While Piko blends into the background and behaves similar to a cardboard cutout, Miki seems larger than life and positively vibrant to those around her. Miki's appearance lends to an obvious understanding that she's a robot, what with her bare arms and legs with joints on full display. So despite her behavior, it's obvious what she is right away. Piko on the other hand, has joints that are harder to see (you really gotta squint at his boxart, for example) but his behavior makes it impossible for him to be anything but a robot (in a world such as this where robots exist, anyway, even if I do imagine they're uncommon in this Vocaloids-are-humans-on-singing-contracts world of mine). So, funny enough, neither passes as human, but for opposing reasons!
Another fun tidbit is that I like to imagine that even if Miki wears clothes that cover her joints, there's just something.... a little bit off about her, that clues into her mechanical roots. Her stare might be too glassy at times, or she may sit too perfectly still if left without stimulus for a while. Just small quirks that mark her as just a little odd, that little bit different. It's nothing huge that you'd notice right away, but spending enough time around her would make them really obvious, if one cared enough to look.
Also I see her as extremely protective of her little brother, tying back into her personality basis in the first paragraph. Due to some behind the scenes troubles, she and Piko ended up being "finished" by different companies, and it's incredibly evident that Piko's company doesn't care that much for him. Miki feels a little bad about it, considering her company puts time into her for system updates and occasional software upgrades vs Piko essentially being finished and dumped. While she is overwhelmed and giddy when he starts to slowly become his own person, she has always been aggressively vocal that there is nothing "wrong" with him, functional emotional core or not. She was fully prepared to love him for the rest of their lives with him never having a single flicker of emotion. While it goes entirely over Piko's head for the longest time, Miki is at least half the reason he's left alone by even those wary or scornful of him. Her loud spunky attitude calms to something borderline motherly when she's looking after her brother, and as he slowly gains emotional intelligence, he's hit by how much she put into taking care of him, and eternally grateful. I'm soft for gentle sibling dynamics ;;A;;
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