The last remaining Luminoth on Aether. The rest of his species is dead or frozen in cryostasis. When it became clear to the Luminoth that they were fighting a losing battle against the Ing, they sealed themselves away to await the warm or cold hands of fate, whichever they might be.
U-Mos is left awake so that he might take care of the Luminoth in stasis, acting as a guardian protector for his entire race. Luckily for him, Samus Aran showed up on the tail of a distress beacon and was able to accept the charge of saving Light Aether from certain destruction.