Question: The title of the character Shadow the Hedgehog is *The Ultimate Lifeform*. As for a translation of this, I *ultimately* decided upon "Ens Ultimatus." But, should "ens" be masculine or neuter? It seems to describe a particular masculine being, but perhaps it should be neuter to describe him as compared to other organisms of either gender, usually neuter as being animals. Basically, is a being expressed as neuter because it is merely an existence of a thing, or is it masculine because of the aforementioned being?

I'm not exactly dying to hear that it doesn't matter, so feel free to delete this question if there are no usages of this word to describe a person or personified being.

PS: I'm back!

Answer: 
*Ens ut nomen* and *ens ut participium* are found in medieval works, used in both metaphysical and descriptive senses: "existence or reality thought of as things" as opposed to "existence or reality thought of as action/process".
But, as an attempt at classical Latin, your version looks completely wrong to me.

In classical Latin the verb *sum* famously has no present participle although, had it existed, it would very probably have been *ens, entis*, which is echoed in, for instance, *praesens*. Lewis & Short quote *ens* as "a thing; formed, like *essentia*, after the Gr. *ousia*, by Flavius (or Fabianus), [and] acc. to Quint. 8, 3, 33 its first recorded appearance was in an attribution by Priscian to Caesar." Smith and (earlier) Riddle, their predecessors, follow Forcellini with something similar. All refer, I believe, to the first recorded usage from classical literature of *ens* in the sense *ens ut participium*. I can't give chapter and verse for this without a deal of research, but the source may be Caesar's supposed regretting the lack of a present participle for *sum* in his lost treatise on grammatical analogy. It seems to me quite wrong to rely on this single reference; perhaps the better word to use is *animal* or even *animans*, as Ovid (Met. XV, 90) has in *alterius animantem animantis vivere leto* (there are similar usages in other authors).

The adjective that you were thinking of is not *ultimatus*, but *ultimus* (the superlative of a supposed obselete word *ultus*). However, *postremus*, the superlative of *posterus* is nearer to what you want.

I suggest that a more apt translation would be *animans postremus*, for which you can vary the gender according to your actual context.
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The word *ens* is a present participle of *esse*.
That tells what the word means ("being", "something that is", "thing") and how it's declined.
The declension is that of a participle: all genders look alike in singular nominative, so *ens* could be any gender.

Both genders are grammatical.
The question is which one is a better semantic fit.
Either one is possible, depending on what you want to emphasize.
I vote for neuter.
The best analogous word I can think of is *chaos*, which is neuter and could be used to mean something close to "the ultimate lifeform".

Choosing neuter seems to emphasize the primordial and shapeless nature of the being, if that's accurate here.
If it's closer to a human or any other typical animal, then masculine is more appropriate.