After reading the comic, I remember about UNMASKED poem which is also Ulquiorra's poem that I conclude it could somewhat relate to this backstory chapter. In addition, I also took the reference of MASKED poem because after all, those books are coexisted for one WHOLE Arrancar Arc. Even their poems are parallel in in rhythm with each other.
That which is masked,
Weakness and truth.
That which is lost,
That which is unmasked,
Desire and nihility.
That which is lost,
While I'm unsure whether MASKED poem would refer anything about Masked!Ulqui or not, since I thought the poem is about the Visorded at first. However, the last line in MASKED talked about "eternal rest is lost". Relatively, grant that the kanji of "rest" (安息) in MASKED poem is different from the kanji "rest" (休) Masked!Ulquiorra talked about. The "rest" in UNMASKED seems likely to refer about the "peaceful death" while Ulquiorra refers more about "sleeping". However, BLEACH is the story about the death, most of the cast are already dead. I guess, "the rest" more likely refers "the peace" in the mind or the soul or the heart.
If we related the context in Ulquiorra's backstory, when Ulquiorra sunk himself in the brush, he thought that he could have an "eternal rest" as in "losing his line of vision", "dissolving into the void", "feeling as if everything had disappeared"... only to find out that he's still alive and he still sees "nothing". You can say that his "eternal rest" is also lost, especially when later he chose high-speed regeneration as his best ability instead of any offense ability.
Now, about his own UNMASKED's poem, the implication seems to be clearer. Ulquiorra talked about "that which is unmasked" implied he might talk about his Arrancar self when he is unmasked or his mask is broken. What Ulquiorra represented as an Arrancar is "dersire and nihility"... exactly refers to two respective poems of his in two manga volumes, one (volume 22) mentioned about "nihility" and another (volume 40) mentioned about "desire".
Ulquiorra's poem in volume 22:
There is no meaning in our world,
neither is there any meaning in us, the ones who live in it.
It is then meaningless
for us, who are equally meaningless, to conceive the world in our thoughts
despite knowing there is no meaning to be found.
Ulquiorra's poem in volume 40:
I envy because of the heart
I'm gluttonous because of the heart
I covet because of the heart
I'm prideful because of the heart
I'm slothful because of the heart
I rage because of the heart
Because of the heart
I lust for everything about you
The last line of the poem seems to be the answer for Ulquiorra's fate, "nothing is lost." Of course, there's various ways to interpret, because after all, we can only take the information of Ulquiorra up to The Ash/heart chapter, we honestly didn't know what would happen to his fate afterward.
In realistic perception, it means literally, "nothing is lost" means "he didn't lose anything" for both nihility and desire because in fact, he did gain "something" that is the "heart".
In optimistic perception, when it talked about "nothing", it means the "nihility" - the "nothingness" (as it implied in the backstory), so in this sentence, it might mean that "nihility is lost" so the only left is the "desire". Granted that interpretation is also plausible.
In pessimistic perception, transition-wise, "nothing" not only implied about the "nihility", but also implied about the "happiness", so in this sentence, it might mean that "happiness is lost". Hence it means both "nihility" which once was his happiness when he's masked and "desire" which is supposed for happiness are lost. Grant he's gone and turned to ash in The Ash/heart chapter. His story as a heartless Arrancar might end here. However, the fact that he gained the heart remains unchanged, giving Ulquiorra a vacant seat for his return. Maybe the story would open another path of Ulquiorra who will not be heartless anymore.
On cue talking about "happiness" [幸福] (koufuku), so far, among Kubo's poems written for Bleach character, there's the only one character that he addressed in his poem about the same kanji of "happiness".
That's Rangiku's poem in volume 46:
It is not terrifying
to know sorrow.
to know you can't go back
to happiness you could have.
In Rangiku's case, she wasn't afraid of "sorrow", but afraid that she couldn't go back the "happiness" she could have. It's as if when the "happiness is lost", Rangiku wasn't afraid of the sorrow caused by the pain of losing it, but afraid that she couldn't get it back the way she could have. The "happiness she could have", I assumed, was the happiness she had with Gin during the time they were together in the past.
If I try to relate Rangiku's poem with the happiness that Ulquiorra addressed...
In Ulquiorra's case, his aspect is nihility, he chose to have nothing to lose, because (as I assumed) he's afraid to experience the sorrow of that loss... for he had already experienced it (with his comrade and the brush). It's plausible to think that once he experienced the *happiness* from the brush of void, he would be afraid of losing it... because it also means losing himself as who he is, a hollow who had nothing. Also, the "happiness" can imply the time he shared together with Orihime. It might speak the volume of his desire to keep that happiness that he was afraid that he would lose to the heart that Orihime talked about, hence he tried to deny her logic with everything he could. Oh Ulqui ;A;
For one, I didn't dig deeper for the meaning of "happiness" in Bleach term, because the concept that Kubo just introduced is too new for me to have any solid facts to base on. So far in Bleach plot, I concur that we haven't yet reached any degree of happiness, not even close to it. Oh please, right now, the angst is everywhere in the story. Like love, happiness can be also misconceived. Like when Ulquiorra thought the moment he sunk his body to the void, he would feel like *happiness*, but it's not.
So far, I don't think the moment Ulquiorra had on the dome with Orihime is the moment he found his happiness. Just as he did admit that he started to interest in her. Just as he implied that he wanted to live, but couldn't go on. Just as he wondered about the heart through Orihime reaching back at him. I can say that Ulquiorra found the heart and his connection. However, happiness!? It's too doubtful and hasty for me to come to such conclusion. WE DON'T KNOW WHAT HAPPENED TO HIM AFTER HE TURNED TO A
Remember the statement when he described about *happiness* "Even I had lost my line of vision, and dissolved into the void, and felt as if everything had disappeared.", while it does fit with what happened to him on the dome, but in his backstory, we CONTINUE to have his eyes open again (regain his vision) only to see ORIHIME. That's why I thought, if Orihime is his happiness, then ALWAYS seeing her should be his happiness also. Hence, he must return to experience it, the heart and then happiness also.
This backstory gave me a sorrow feeling for Ulquiorra's past and his nihilist character, but it also gave me hopeful feeling toward the continuity of his story (involving with Orihime) since there's no closure, yet begging more mysteries and new concept for his story. My analysis ends here. Thank you for putting up your patience to read such long rant.