"It just kinda, uh... It hurts not being noticed, is all."
-Yuuka to Mathew, after the Praetorian attack.
Yuuka's parents adored each other, though Alex was a bit hesitant to be more open about his own thoughts. For the first time, they had someone to bond with about their difficult lives; for Mami, the terrors of the life of a Puella Magi and for Alex, the horrors of being stalked by an interdimensional entity. In this timeline, considered a failing timeline by Homura Akemi, Mami stayed alive much, much longer. In fact, she survives until the age of 16, and at that point, Alex was about 25. In the two years that Mami and Alex knew each other, Mami took Alex everywhere she could, even to a Witch's barrier if she knew it would not be too dangerous. In this timeline, Mami's wish was that she could have but one friend, with whom she could share all her deepest desires with, and who could make all of them come true. It was not sexual reproduction that birthed Yuuka, but that wish. Mami's death was not necessarily a "death", but she fell into despair when Alex began to purposely drift from her and push her away, for what he thought was her own safety and happiness. It was also at this time that Alex's sanity was fading, and he could not bear the thought of forcing Mami to deal with him. She became the Witch called Bertha, taking the form of a morbidly underweight matron figure in a long, ripped blue dress, having long arms and hands for eyes, with pitch-black, taut skin and balding blonde hair. It was also notable that Bertha's form had an unusually bloated stomach, perhaps a symbol of childbirth and commitment. Sayaka Miki and Kyoko Sakura teamed up to defeat Bertha. However, as Bertha disappeared, she did not leave a Grief Seed, but a miniature Soul Gem, which was given to Alex to take care of. He reportedly cried upon seeing it. Because of this, Yuuka has the capacity to become a Magical Girl, but she lacks the resources since Kyuubey will not find her any time soon. If Kyuubey were to find Yuuka, though, she would probably not be the most powerful of Magical Girls, but she would be a legendary Witch when she fell into despair. Yes, it would be a matter of "when", not "if".
Alex awoke to quite a shock when a mere week after Mami's official death, the drawer Alex stuffed the Soul Gem in it began to rattle and fuss! Alex tried his very best to raise Yuuka well, but it was a rather sheltered environment, until she was about 11. At that point, he left her care to his old friend Jay , who he called her uncle. Jay was arguably a much more enjoyable guardian for Yuuka, he actually kept her in the loop of things whenever he could, and he was easy to approach with most things. However, Yuuka still misses her "biological" father and hopes they can meet again someday. As Yuuka never met her mother, she does not have much of an opinion of her. She was never told anything about Mami either, everything she knows about her is from Jay.
Yuuka quite clearly feels overwhelmed by most of the others. She has mentioned feelings of worthlessness and fear to be open. She seems to desire a closer relationship with everyone, like they all seem to have with each other, but she does not seem to want to subject them to dealing with her. However, she seems to have placed a degree of trust in Siren and Xavier . Since she tagged along with the group, she has clung to Xavier for the purpose of having a "translator" of sorts, as she is easily lost and confused. She seems very, very close to Xavier, to the point that she can feel at a legitimate ease if he is by her side. Recently, she has confessed romantic feelings for him and they are now a couple. She consulted Siren for help with confessing to him, and she considers the bat to be an ideal friend. However, she is still too shy and afraid to tell Siren how she feels, and is mortified at the idea of being ignored or brushed off.
Yuuka is overall a very jittery and insecure person, she has nothing good to say about herself. She is desperate to stop feeling this way, and seems to be looking for a way to stop being useless, like she thinks she is. Though it is true that she has done nothing truly earth-shattering, no one seems to actively think of her as useless. She symbolizes the "average self-hating teenager" archetype and also the desire to grow up quickly. She wants to be big and significant like she sees Piper as, but she's a bit unlucky in this area. She has a need to be included that is rarely satisfied in her own eyes, but she does not harbor very much animosity towards the other Mistakes.
She also has a separate intelligence that none of the Mistakes know about, and it is dormant in her at most times. This other intelligence calling itself "The Bride" treats itself like a counterpart to Xavier's alter-ego, "The Bully". The Bride is quite egotistical and cocky, carrying Yuuka's body as if it is the height of sex appeal. This separate intelligence represents Yuuka's desperate need to protect herself from The Bully lest it shows itself again, and even the desire to protect Xavier from it.