"aster" can refer to a number of characters, artists, and other things. Check the current list of `aster_*` "tags":/tags?commit=Search&search[hide_empty]=1&search[name_matches]=aster_*&search[order]=count to see if there is a more specific one that fits what you are trying to tag.
Some of the more popular tags from that list include...
Artists
Characters
Misc
...but check the "complete list":/tags?commit=Search&search[hide_empty]=1&search[name_matches]=aster_*&search[order]=count if the one you want isn't listed here.