Dustin

Many years ago, Dustin came to Senanonic and joined the Citadel as a medical expert alongside his significant other, Melanie, a botanist. In commemoration of their wedding, the married couple planted a cherry tree in the greenhouse on the Citadel's airship and joked that once their child grew up and met the love of their life, they would gift the fruit of this tree to that child.
Sadly, while on a field trip, Melanie was attacked by Chaos Remains and had already lost consciousness from her injury when she was rescued. Her life was saved with the help of the Citadel's medical technology, but she fell into a deep coma. To heal her, Dustin practically tried anything he could think of, even wanting to participate in an exceedingly dangerous project for a time despite the price of it being losing his reason and past memories.
Right as he was stuck in a dilemma, Melanie miraculously regained consciousness, albeit only for a few short minutes. Her feeble fingers rested on his palm the way they had held hands for umpteen times, and her last words for him were, "Don't forget me."
Following that, all that filled Dustin's heart was hatred for the Chaos Remains that took his wife's life. For the sake of seeking vengeance, he participated in another project and accepted some physical modifications, turning from a doctor to a soldier who made waves in the Citadel for his radical, high-risk combat style. In the beginning, he merely had his four limbs substituted with prosthetics, but as the injuries he got from various missions multiplied, the Citadel had no choice but to successively substitute his skin, muscles, skeleton, and even part of his internal organs.
For a long time afterward, Dustin wallowed in endless hatred and sorrow, becoming a beast whose lone goal was to tear apart all Chaos Remains.

To the young Citadel members, Dustin's appearance bears no semblance to that maniacal avenger noted in records years ago, whereas people familiar with Dustin generally believe the change in him only began after the emergence of a little girl. Many years ago, Dustin joined a rescue team and headed to a village destroyed by Chaos Remains, saving part of the survivors. One of them was a child named Sommer. Following that, he returned to the lifestyle of fighting his life away on battlefields, while Sommer, with her unstable Ascension Ability, was sent to the Citadel's medical department for long-term observation and treatment.
The two should've never met again... but sometime later, Dustin reached out to the Citadel and applied to adopt Sommer. Everybody was shocked by the maniacal avenger's sudden interest in raising a child, but on account of his past qualifications and character, the Citadel ultimately approved his application.
"Mr. Dustin may be made of metal from head to toe, but he is a very warm-hearted person!" is something plenty of Citadel kids would say these days.
Once in a blue moon, when Dustin does return to the Citadel's airship and gets surrounded by the kids, he will suppress the hostile aura he directs at enemies on the battlefield and willingly answer their every question.
"How did you and Sommer meet, Mr. Dustin?"
"Haha, she helped me take care of the greenhouse left behind by Melanie for a long time... By the time I learned about it, the plants in there had already been revived. Truthfully speaking, for a long, long time, I forgot that I still had places to go. Sommer was the one who snapped me back to reality."

When Sommer brought the severely injured Nayuta back to the Citadel's base, Dustin was the one who applied emergency first aid to Nayuta. With the Citadel's technologies, Nayuta's injuries weren't considered life-threatening, and the young man's breathing stabilized. Dustin wiped off the blood stains on his mechanical hands and quietly asked Sommer to talk with him outside the hospital ward.
"I won't ask you to be responsible for helping an unknown child. All I want to know is, why did you help him?"
"Because... he's injured... and..."
Sommer found it really hard to answer this question. Seeing his daughter in this state, Dustin suddenly recalled the moment when he met her again.
At the time, he had returned severely injured from a dangerous field mission and had no choice but to lower his core operating power and lie on a hospital bed in the waiting area. Coincidentally, a group of children waiting for their turn for physical checkups passed by, the majority of them steering away in fear and refusing to be near this mechanical man soaked in blood.
Sommer was the only one out of the group who trotted over with fear and slight anticipation, holding Dustin's hand like she was soothing his pain.
...
"Never mind. I trust your judgment, Sommer. Let's wait for him to wake up for now."
Sommer decided to go back and watch over that young man, while Dustin came to the greenhouse left behind by Melanie. In recent years, Sommer had taken great care of this place, and the plants were growing with much more vitality than before.
Melanie's unfinished last words replayed in Dustin's mind innumerable times throughout the years, but he never once figured out what she was truly trying to say. He knew she wasn't referring to hatred, but aside from seeking vengeance, he had no idea what else he could do.
Dustin gazed at the sunlight shining through the greenhouse's glass. In the warm light, he recalled Sommer's smile, that deathly pale young man, and the hands he had held umpteen times belonging to the child he saved.
"Don't forget me..." and live on to save many more.