Harem fem Harry x Ririka x Misa x Luca(you could add more)
for more crossover potential Lagrange the flower of rin-ne, kiddy grade and Darling in the Franxx
MOD Fem Harry (Cordelia) at the age of 23 she goes through the Vail of Death (you choose the reason) and ends up a few years before gunbuster where to fit in and learn more about the world she disguise herself as a 16/17 year old girl and joins the school by the time the plot starts she is the number one pilot and her age, everyone learns that she will mentor the chosen 2 maybe during the first mission Ayamo was about to get hurt but Cordila sheilds her and gets fatality hurt while on the way back everyone is worried but her wound heals at a visible rate and her secret is out but they decide to keep it a secret not wanting Cordila to be sent on suicide missions. After that the girls could come to see her as a mother/sister figure and after the end of gunbuster they could become immortal(she could blood adopt them) as well so she's not alone they could use magic to make new identities with the help of someone from the government maybe a decendent of one of the og group. If you read the Italics and decide you want to do one of those first Cordila could be a famous pilot with her daughters/sisters alot of people look up to them as a role model though eitherway they learn/watch about her maybe she could repeat what she did with Noriko and Ayamo and mentor the protagonists. If you do one of the others first before BSP Cordila could adopt everyone(protagonist) from the other show(s) doesn't matter whether their her kids or siblings they needed her so she was there(she curses not only her potter luck but her saving people thing) her parring has to win over her overprotective kids/siblings this would be even funnier if she is a pirate who is known for taking(kidnapping) those in need and (not blood)adopting them so her whole pirate crew/company is filled with people she adopted