The Life aspect is some kind of innate "life force" of living beings, necessary and sufficient for life to exist. Life players displayed the ability to heal teammates by repairing grievous wounds and have even been shown to be capable of resurrecting themselves. Interestingly, going by Feferi and Jane's examples, Life powers appear to glow in the player's text color. The Condesce was able to extend lifespans, and it is shown that her pre-scratch self, Meenah, is a Thief of Life. If Doom is associated with death, then it may very well be the antithesis of Life.
An alternative theory for Life is similar to the theory that Doom is associated with system rules and structure. As Doom may be associated with understanding the rules, Life may be associated with testing and subverting them. Biological life is known for its resilience – one need only consider organisms that live next to volcanic vents at the bottom of the ocean to appreciate this. Life adapts to find a way to survive, and in the process, probes the extremes of possibility. If both theories are correct, Life would remain opposed to Doom as expected.
Meenah demonstrates a very experimental mindset with regards to the way things work (speculating on concepts such as whether it's possible to "double die", as well as wanting to stab Roxy just to see what happens), and as the queen of Derse, the Condesce is perfectly happy to bend the rules by endorsing "tactical shortcuts" in the war against Prospit. Being described as Sea Hitler, Condesce was the one who upheld draconian laws on Alternia, that have cost countless lowbloods lives. As an interstellar conquer leading an army of murderous troll adults she was accounted for taking billions of lives. On Earth she drove humanity to near extinction. But, being a Thief of Life, Condesce was only able to take lives - any attempt to revive her race (giving life - the exact opposite of her role) ended up in vain.
Known Heroes of Life are Jane Crocker, the Maid of Life; Feferi Peixes, the Witch of Life; and Meenah Peixes, the Thief of Life. Jane's denizen is Hemera, and it is speculated that Feferi and Meenah share this.