- "Cassie is a natural estreen -. a person with an ability to make morphing almost artistic. On my planet it is an art form. There are professional estreens who change shape in fantastic, beautiful ways. Cassie was not a professional, but she had the talent. As she morphed, she formed pleasant shapes."
An estreen is an individual who is naturally gifted at morphing. Some beings with the ability to morph are able to control the morphing process in a way that makes it look beautiful rather than disgusting and are better able to endure the strain of repeated morphing over a short amount of time. Cassie has demonstrated this talent.
There appears to be no biological explanation for this phenomenon, it seems to be merely an inherent talent. Estreens are predominantly females, although male estreens are not unheard of. Some evidence suggests the psychological makeup of an individual also plays a role. It is theorized, however, that under extreme stress and with great motivation, that one who is not an estreen may be able to control the morphing process in some way.
Andalite estreens can make a living based on their talent. For example, Estrid's mother is a morph dancer and Ax has mentioned morph artists. While the talent is considered mostly aesthetic, it can be useful in rapid morphing or in morphing clothes.