in the anime:
tin nyanko wanted nothing more than to be the number one animamate and have mau as her own planet (although she never says that she came from mau). she was the last animamate to appear and the last one to get killed by galaxia. tin nyanko would do whatever it takes to get the star seed for galaxia. when she first appeared, she disguised herself as suzu nyanko and enrolled in the high school as an exchange student from libya. she was half healed by sailormoon, so her outfit turned half white, but she was killed by galaxia before the process was completed. *sniff* but during the time she was half healed, she lost one bracelet, but she could still come and go using the telephone box.
tin nyanko is probably the most famous for killing sailor lead crow. but even before that, she was actually responsible for aluminum siren's death as well, because she convinced galaxia to do it, and then galaxia felt motivated and killed aluminum siren. this only made lead crow hate tin nyanko even more, though, because crow didn't have her very best friend after that. ^_^
in the manga:
tin nyanko was much more signifigant in the manga. she also enrolled in the high school as an exchange student from libya with the same name, suzu nyanko. but she also warned her away from the sailor starlights, saying that "girls who dress like men can't be trusted!" nyanko also asked usagi to play with her at usagi's house. later on, luna and artemis detected her, since she was from the planet that they were.
so when they confront her about being sailor mau, she screams that there is no sailor mau and that the planet mau is destroyed. then nyanko faces off with luna and artemis. nyanko accused them of being traitors and leaving mau. so then she tried to blast luna, artemis, and diana with her galactica puppet attack, but eternal sailormoon came along and blocked it. but eventually, nyanko was able to scratch all three of their moon symbols, turning them into cats. la!!! >^-^<