Aster is a young boss monster, being a skeleton/goat hybrid, before becoming an echo flower.. He is the son of both Toriel and Sans, as well as being the twin brother of Verdana. Aster, much like his sister, can remember all the timelines he existed in. Aster can somewhat recall the original timeline.
Aster is shown as a generally cold and always angry monster to those he is not close with. He is usually NEVER seen with a smile on his face, and almost always scowling or glaring. In the inside, Aster is aggressive, however is more open and caring.
Because Aster only trusts Verdana truly, it's rare to see him open up to others, especially his own parents.