Are the Endless alive? For that matter, are the Omicrons alive? Let's get incredibly granular about it and find out! For something to be considered "alive," it must meet the seven criteria of life. These are, pulling from Wikipedia:
1)Homeostasis: regulation of the internal environment to maintain a constant state. I think an argument could be made that Endless adhere to this, though somewhat abstractly. The Omicrons would also pass this test, as they need to maintain a correct internal environment to function.
2)Organisation: being structurally composed of one or more cells. Endless are composed of aether, so they don't pass this one, unless one wants to argue that aether can be formed into functioning cells. The Omicrons fail this one as well, as they have fully inorganic bodies.
3) Metabolism: transformation of energy. Living things require energy for homeostasis and other activities. While they don't meet the strictest definition because they do not anabolize or catabolize matter, both the Endless and Omicrons do need to take in energy to keep existing, and depriving them of energy would more or less "kill" them.
4)Growth: a growing organism increases in size and structure. The Endless don't meet this criterion nor do the Omicron because they do not start out as a single cell which then divides over and over until it has developed into an organism. They are constructed and built, not born.
5)Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat. One could argue that the Omicron possess the ability to adapt themselves to new environments, though it seems more artificial than natural. I don't believe anything suggests that the Endless could do this, however.
6) Response to stimuli: Both the Endless and the Omicron pass this one, they have senses that allow them to interact with and react to their environment.
7) Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms. We don't know anything about how Omicrons make more Omicrons, but we know they can. Granted, this is probably just a matter of "build more robots" with no DNA involved, and thus they would not qualify. The Endless definitely can't reproduce because they have no DNA to reproduce with. They cannot make more of themselves, they can only wait for more memories to be uploaded and given aether.
So if I'm being very generous, the Omicrons score a 4/7 on the Alive scale and the Endless score a 3/7, making the Omicrons slightly more "alive" than the Endless, though they both don't actually meet the scientific criteria of being alive at all.