The Makajiu

The Makajiu story arc was the only one that was not in the Bishoujo Senshi Sailormoon manga by Takeuchi Naoko, and also one of the shortest arcs, second to the Neherenia arc in Stars.  It was about a tree from outer space and the aliens that were born from the tree, the last of their kind.  They had come to earth to gather energy and feed it to the tree, because if the tree didn't get enough energy then it would die, and then the aliens themselves would die.

Ann in alien form
Ann is the female half of the alien family.  She disguises herself as a human named Ginga Natsumi at Juuban Junior High and is in the same class as Usagi.  She's sort of hypocritical (but then, Ail is too a little) in that she says she doesn't like humans but then she falls in love with one, Chiba Mamoru.  Well, she doesn't really love him, more like infatuation.  She doesn't respect authority (such as Haruna-sensei), which is probably because she just doesn't like humans.  Whenever Ail calls up a cardian, she has to choose it for him or else it won't work right.  She and Usagi are rivals for Mamoru's attention, because Usagi is trying to return Mamoru's memories.  She has practically no patience and/or tolerance for anything, and she can be bitter and sarcastic often.  But she is also very brave, protecting Ail from the Makajiu's branch at the cost of her own life.  But she doesn't die permanently (like most normal people do), after Usagi heals the tree she is restored back to life, and she and Ail go off into the galaxy to plant their tree somewhee else.

I actually like him better in human form
Ail disguises himself as Ginga Seijuurou, a human in 10th grade at Usagi's school (he and Ann are disguised as brother and sister).  When he gets there, he practically develops a fanclub of female students (for obvious reasons), including Makoto.  But, unfortunately for his fans, he becomes infatuated with Usagi, possibly because he is drawn to her positive energy.  He is usually a nice person; he's generous, hospitable, and sort of passive.  He is also romantic, and he understands love better than Ann does.  But he also doesn't like Mamoru, of course, because it's he that Usagi is always fawning over (even though at this point in the series Mamoru doesn't have his memories back).  Ail is also conniving and vengeful, especially towards Mamoru.  He sends a Cardian after him, but the Cardian doesn't obey him because Ann didn't help him pick that card (see, she has to pick it and then he summons it with his flute, or else it doesn't work very well and the Cardian turns out to be a - pardon the pun - wild card).  But in the end, instead of letting Ann kill Mamoru, he tells her to spare him because he sees that Mamoru and Usagi love each other and that destroying that would be wrong.  He eventually does realize that he loves Ann, when she is killed (well, that's a convenient time to realize something like that), and when she's brought back to life they both take the Makajiu seedling and leave Earth to plant it someplace else.