Here's another sample pack of custom Mikata skins, courtesy of the unofficial maintenance project. There's a lot of these floating around, I'm told. I guess people have had a long time to make 'em.

TIRED.pk
Added in official patch 1.4. Appears when the user has been logged in for at least 48 consecutive hours.
COMMENT: "Apparently, they added this one to discourage bad usage habits, like leaving your machine on for a long time. Never officially confirmed, but we get a lot of topics on the forums asking how to prevent it from appearing. People say they feel bad for her when she looks like that, so I guess the strategy works. Restart your PC and she'll be able to squeeze a nap in."

HOLIDAY.pk
Added in official patch 1.4. Appears on Christmas Day (25/12) between 00:00 and 23:59.
COMMENT: "Is she meant to be an elf or a Santa? People have gotten banned from the community over this question, believe it or not - something about it seems to provoke strong feelings. She makes little sleigh bell sounds when she moves around with this one, which is as adorable as it is obnoxious. Helps you keep track of her when she sneaks behind a window, though."

HALLOWEEN.pk
Added in official patch 1.4. Appears on Halloween and the following day (31/10 - 01/11) between 18:00 and 06:00.
COMMENT: "According to one of our daily polls, 41% of users say that this skin's 'devious expression' is their favourite thing about it. I prefer her little fangs, but to each their own. Supposedly, she can turn into a bat and fly around the screen, but nobody's ever seen that happen. We could decompile it and find out, but the other members of the team agree that it's more fun to leave it as a mystery. Keeps with the theme."

EXPLORER.pk
Added in unofficial patch 4. Rarely appears when the user opens a recognised web browser.
COMMENT: "Thanks to UltoSpark for the art on this one; it's damn near official-looking. There wasn't actually a ton of work to do with compatibility in the first few patches, so we decided to spend our time making some new 'secret' skins that'd only show up once in a while. What really surprised us is that Mikata can actually use accessories attached to skins, even though none of the official ones have anything like that. Occasionally, if she isn't nearby, she'll pull the binoculars out and take a look at what you're typing or what your cursor's doing. Funny how she still surprises us sometimes."

INTRUDER.pk
Added in unofficial patch 6. Rarely appears when Mikata is accessing files or folders in the 'Users' directory.
COMMENT: "Anyone who's used Mikata for more than ten minutes knows that she likes to pry. There's a running joke on the forums that she's a bit of a stalker, so team member 200Waggler threw this skin in with patch 6. That was a few years ago, but it's still funny when she turns up in this skin and reminds you that your Pictures folder is in good shape, as though she hasn't rifled through it at least twice a day since you installed her. Like with the last one, she'll point the flashlight at things, but we're not really sure how she decides whether or not to use it, or what to use it on. Good for a laugh when she points it at the cursor like she's caught it trespassing."

GOWN.pk (aka: FANCY.pk, EVENT.pk)
Unknown origin. Rarely appears between 18:00 and 02:00. File name is not consistent across uploads.
COMMENT: "I don't know where to begin with this one. Non-standard colours? Makeup? Lighting effects? Don't get me wrong, I like it, but whoever made this one clearly has a different idea of how Mikata should look. The trigger conditions are pretty obscure, too. Most of our 'secret' skins just have a small percentage chance to show up when their conditions are met, but the list of conditions for this one makes up a solid half of its decompiled file-size. I guess only Mikata really knows when it's the best time to dress up."