Lucina "Brawling Lucifer, Nagu-chan, Nagu-Lucifer, Lucy"
About
She debuts in the second arc. Despite being a thoroughly intimidating fighter and individual, Lucifer is a devout caregiver who wholeheartedly works with other kids at her parents' cram school. Taking it very, very personally when any of the little kids her family looks after ends up hurt, saddened, or dead at any point.
Lucifer loves kids, especially the schoolgirls whom she and her kin look after. Going on a tirade with the intent to find whosoever does harm unto them. She was the only other person Geeko could associate with while being her crazed self for a time. This changed as the two got older, but they're still very close.
One of the revived corpse soldiers belonging to Nero's faction in later arcs was a child killer who prowled around Lucifer's neighborhood. The assailant took his own life before a trial could get underway and properly convict him for his misdeeds during incarceration. Lucifer was abjectly livid at never getting the chance to exact revenge against him, as one of his last victims was someone she deeply cared for and loved tutoring before they died.
(Source: TVTropes)