Yūichirō Hyakuya (百夜 優一郎, Hyakuya Yūichirō?), birth name Yūichirō Amane (天音 優一郎, Amane Yūichirō?), is a character in Seraph of the End. The main protagonist of the Seraph of the End: Vampire Reign manga, Yu along with Mikaela Hyakuya, possesses the "seraph" gene.

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In Yu's demon form, he
alternates between
earnestly spoken deception
and furiously voiced threats
to get what he wants. Attempts
to persuade Mika into freeing
him include enticing him
with his blood, and promises
for them to run away together
if only he would set him free.
With him not being freed from
his restraints, Yu wants to
kill and destroy anyone nearby.
He screams and rages that he
will devour their souls. When
Yu regains his human mind,
he does not recall any of his
actions as a demon. Despite
this, he appears utterly
nonplussed about losing
his humanity.

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❖ ... According to Asuramaru, he is able to think much more rationally without all of those emotions in the way. While Yu could consider allying with vampires they could not defeat, and whose information they needed, Yu was instantly swallowed in a murderous rage at seeing a deeply unsettling sight.

After receiving the
medication, Yu's
personality returns
to how it was before
he became a namanari.

While drawing on demon power with demonic horns and fangs himself, Yu can still retain his sanity despite his murderous outer appearance.

⛓‍💥 {– ′} In his demonic state, Yu would express the intent to kill Guren no matter the cost. ♠︎

My dearest Mikaela,

As I sit here and write this letter, my heart overflows with gratitude and love for you. From the moment we first met, you've been more than just a friend; you've become my family, my brother in every sense of the word.

Remember when we first found each other in that dark place? You were a beacon of light in the midst of all the chaos. Your kindness, your strength, and your unwavering loyalty gave me hope when I had none. I knew then that we were bound together by something deeper than blood.

       Over the years, we've faced countless challenges together. We've laughed, cried, and fought side by side. In each victory and each setback, your resilience has inspired me to keep going. Your unwavering belief in our cause has given me the strength to never give up, no matter how bleak things seemed.

       But beyond all of that, Mikaela, it's your heart that I cherish the most. Your compassion for others, your willingness to sacrifice for the ones you love - these are the qualities that make you truly extraordinary. You've taught me what it means to be family, not just by blood, but by choice.

       So, as I write these words, I want you to know that I am forever grateful to have you in my life. You are not just a friend or a brother; you are my family, my home. No matter where our journey takes us next, I know that as long as we are together, we can face anything.

       Thank you for being you, Mikaela. Thank you for being my family.

With all my love and gratitude,
Yuuichirou Hyakuya.

Dear God's most beautiful creation,

In the quiet moments of twilight, where the world seems to pause and breathe, desires emerge like delicate whispers carried by the wind. They are the ethereal threads that weave through our dreams, shimmering with the allure of what might be. These longing thoughts, resplendent and unspoken, paint our inner canvases with hues of hope and reverie. They dance with a grace that belies their depth, reflecting the heart's most intimate wishes and aspirations. In this symphony of yearnings, each note resonates with the promise of possibility, inviting us to pursue the beauty that lies just beyond our grasp.

                            Yours faithfully,
                            Yuuichiro Hyakuya