Sakuya Kumashiro from Tenchi in Tokyo could've been better written, but the logic behind her role as the story's "Relationship Sue" is actually rather interesting... As a Tomato in the Mirror, she was created to be one of these by the Big Bad herself: Sakuya is explicitly mentioned to be Yuugi's modified Projected Avatar, who CANNOT imagine her life without Tenchi and disappears when he leaves her to save the world. She's basically the evil and Dangerously Genre SavvyReality Warper's Mary Sue, and it's vital to the plot that she remains as one. — Orihime and other anonymous troper.
I saw the next-to-last episode of Tenchi in Tokyo first, and Sakuya's actions in that episode made me a fan, but upon rewatching the series I had to admit that, particularly in the early episodes, she was fairly annoying and clingy, verging on tears at the mere possibility that Tenchi didn't like her. Then I came to a similar conclusion to above—she had to be that way, not only to assure that Yugi's plan would come to fruition, but also because she didn't have any life apart from Tenchi; no family or past. If she lost him she wouldn't have anything to live for. Thinking of it this way makes the end take on a different light- it's not Tenchi who has to give up Sakuya to save the world, it's Sakuya who has to accept that Tenchi has better things ahead of him, and that keeping him with her just to keep herself around would be selfish. Another moment of fridge brilliance came after reading the above post... at the beginning of the series it's established that Sakuya is among the most popular girls in school—but in light of the ending, one realizes that Yugi had to have given her that status—because her greatest fear is being alone!
Also another one from Tenchi in Tokyo is Yugi's plan always backfires because she couldn't control her powers completely yet. That's why the monsters were ridiculous or had huge personality flaws, the only thing preventing the plans from working. If: she hadn't been reformed in the end and had her powers fully matured then there's no telling what she can do.
Here's one. If Mihoshi's last Did You Just Romance Cthulhu? relationship broke her mind, how's she going to come out of it when she's one of the only basically normal types in household full of Cthulhu?
Well, you have to remember, she isn't exactly normal herself, what with being incredibly lucky and whatnot.