Aeternalisks are Aeternalae that cling to a serpent nature at least somewhat. The majority of Aeternalisks simply stayed with their original Ovalisk colony after-evolution so long that they retained their Ovie-traits permanently. This gives them, among other things, the ability to do either an Aeternalae Touch or an Ovalisk one, at their choice at the time that they do each Touch.
It has been know for the Aeternalisk evolution to occur spontaneously in rare instances among those Touched by Aeternalisk or Aeternalae. Also, there are rumored to be certain magical items that can store Aeternalae Touch potency and cause it to change those that it is used on into Aeternalisks.
The Aeternalisk nature partakes of the eternal child-like sense of wonder, willingness to believe in the impossible, extreme love of cuteness, and friendliness of Ovalisks. But this is leavened with the ability to think deep and complex thoughts and get angry like Aeternalae (but they still never hate). The nature of the Aeternalae society that they eventually take up with, and the environment that they live in, does influence their overall out look also. Thus you could have, for example, a Mazoku-Aeternalisk that has agressive tendencies interleaved with her friendliness.