Yuki was born in a noble family on the 'High Tech Feudal Japan' world of Shirou III.
From her early years she was very intelligent, agile and cunning. Her parents knew that their daughter is a very gifted child (partially thanks to gene-therapy), so they weren't surprised when one day a lone visitor appeared near the gates of their estate.
That man brought the message from the Death Cult temple. That message was an invitation for Yuki to become one of the Dead Leaves sisters.
To receive such invitation was a great honor for every family on Shirou III. Parents agreed, and prepared a symbolic funerals for their daughter. After the ceremony, Yuki was taken to the temple.
After that event all information about Yuki's life was classified, and only inquisitor Kate Blackwing, having required access, knows what happened to Yuki afterwards. -
Yuki was severely hurt (maybe even crippled). Kate doesn't want to share the story with the rest of her team, so the cause of such traumas is still a mystery.
Yuki's body covered in scars, and her eyes are artificial ones. Previously they were red, but Kate applied her inquisitorial influence, and gave Yuki new, very advanced eye implants. Now her eyes glow blue if she's calm, and bright-red when she's in combat.
Yuki and Mahina are good friends. Mahina was shunned by everyone in IG company she worked in, but Yuki accepted young tech priestess. To show Yuki her gratitude, Mahina looks after Yuki's gear, and sometimes presents her friend with new gadgets and weapons. One of those presents is Yuki's custom-made hellpistol. A deadly, rapid-fire weapon, which cuts armor like a hot knife through butter. -
Yuki is calm and serious. She acts like a 'big sister', looking after the rest of the team (except Vladimir and Subject 27). But she's serious only when circumstances demand it. In 'normal' life she's quite cheerful and talkative. Sisters Ada and Pury adore her, and often spend their time with her. Mary is a bit envy, but still likes her. A-LIC-e sees Yuki as 'one of her kind' and respects her skills. Kate is Yuki's 'little sister', despite her commanding position. Kate always grumbles about 'excessive amount of care' Yuki applies to her, but deep inside she likes it (and Yuki knows it).
Yuki's combat specializations are recon, silent assassinations, kidnapping, sniper shooting, HtH combat and infiltration. She can apply a wide variety of toxins and poisons. In general, Yuki is a 'stealth'-type assassin. She likes to do things quick and silent, leaving no traces, evidences or witnesses.
Your characters all seem...slightly samey to me. Its not bad, but its what would normally be know as a 'mary-sue'. Its because they're mostly slightly mysterious 'don't get them angry but they're really nice people' types of characters. They need to be flawed, and this is an inquisitors retinue, words like 'adored' aren't thrown around in those kinds of confines, or words like 'love'. Hell, even friendship doesn't happen in some of them.
Its mostly because bad characterisation annoys me, and I try to help others improve it. Looking at other criticism to your art, you're not very open to that, so good luck, peace be with you. Sionara.