Humanity has not yet discovered Miki's planet of origin. The opposite, however, cannot be said. A decently-sized chunk of the universe has one-sided awareness of planet Earth's existence, and generally considers it to be an outdated place inhabited by harmless humans that, frankly, need to step up their game.
In part, this is why SF-A2 Miki appears so humanoid despite having been manufactured a considerable distance away from planet Earth. In order to serve as a messenger between her home planet and the neighboring planets threatening intergalactic war, Miki needed to appear entirely nonthreatening. The government scientists behind Miki's development followed the planet's desire to defuse conflict, so they designed their mode of communication to be docile, perky, cute, and kind—anything to try assuaging the hair-trigger tempers of other galactic powers.
Into her pod Miki would go to be blasted away to another world expecting her arrival. Brightly, she delivered the message she had been sent with, and waited patiently for a response, which she would only record upon being given permission. Back again she would go to be shuttled along to her next destination, again and again and again> In between, minimal studies of and expansion to her AI were performed, but for the most part, she would lie dormant in sleep mode until being given another task.
Centuries of hostilities weren't so easily settled, though. Even when the battles begun, there were still alliance to establish, offers to discuss, and negotiations to be had. Miki became the face of all of them. The routine went on—just with more explosions.
It hardly lasted months. On her way through guarded territory, Miki's pod alarmed a trigger-happy new recruit in their new defense vessel. They only recognized her shuttle as a messenger ship after they'd shot her out of orbit.
Having been in sleep mode, Miki couldn't even begin to provide an estimate as to how long her pod, with its broken coordinate systems unable to guide or track her, spent drifting through empty space. Eventually, though, it found Earth's atmosphere, and proceeded to crash land in the middle of bumfuck nowhere, Japan.
The Vocaloids found her first. Programmed to keep secrets, Miki couldn't disclose the details of her origins, which didn't deter the Vocaloids from almost immediately going can we keep her. Nobody intended to build her with a singing voice, it just sort of happened, and Miki knew her own ins and outs well enough to assist AH-Software developers with minor adjustments to make her a better fit as a virtual idol.
The programmers behind Miki's AI focused on friendliness as the most important aspect of her personality. In the midst of high tensions and danger, Miki's cheerfulness should have eased the stress of communication and inspired a sense that everything would be okay. So for the most part, Miki comes across as an upbeat moron. Next to nothing can put a damper on her perkiness; even in the face of being blatantly insulted, she smiles and enthusiastically makes polite conversation.
Now that she's become acquainted with humans and androids closer to the real thing, she has been making an effort to understand emotions on a deeper level, because she wants so badly to be able to connect further with her friends. Their importance to her can't be overstated: she hasn't completely adjusted to being liked, loved, and part of a community as opposed to being used as a tool. Her friends inspire more genuine enthusiasm in her already excitable self than anything, and with her original purpose steadily becoming forgotten and irrelevant, she dedicates herself entirely to their well-being.
Because of her sunshiney attitude, though, the idea of sadness eludes her. She struggles to recognize subtle expressions of sadness, and hasn't the faintest idea of how to comfort a sad person or figure out what's ruined their mood. She recognizes anger much more consistently, but hasn't yet managed to figure out what causes it. The hows and whys of fear come easily to her, as does helping reassure someone who's afraid.
She's picked up a very lofty and idealized image of romantic love from the subpar romantic comedies that have become her absolute favorite thing.
Miki was programmed to remember the messages given to her and recite them word-for-word. If notified by someone else to pay attention to something, whether or not with those exact words, she will remember everything that follows word-for-word and down to the last detail of the room. She's like a walking recorder! Her memory in everyday situations can't compare. She'll lose just about anything you try to get her to hold onto and her personal space is horrendously disorganized.
She speaks entirely in third person: in order to prevent confusion, Miki addressed everyone involved in a message delivery specifically by name. "Miki has a message from X to deliver to Y! In light of Z's previous proposal, X would like to ask Y to consider..." etc etc etc; up to "Miki and X would like to thank Y for Y's time!"