Kingdom Hearts vs Honkai Impact 3rd

Connection:

Both are blue-haired female video game heroines from hair to signature clothing who are some of the most tragic characters in their respective series and battle against a hostile antagonistic force that threatens to destroy everything they touch (Heartless and Honkai respectively) where they can turn people into monster like beings until nothing is left of them (the Heartless is a hostile species that comes from people consumed by darkness while the Honkai turns people into Honkai Zombies), and turn people into people with powerful beings (Nobodies and Herrschers respectively)

Both of them are orphans taken in by a parental figure (Master Eraqus and Captain HIMEKO respectively) who would go on to train them but unfortunately would end up dying

Both of them would become fighters utilizing magical/spiritual powers (Magic and Honkai Energy respectively) and would join major groups/organizations (Guardians of Light and Fire Moth respectively) where they meet and become close with young male comrades who tragically become corrupted by darkness (Terra and Kevin Kaslana respectively) and also younger sibling like cheerful comrades who at one point fell into a deep sleep and wouldn't be awakened until the events of the series (Ventus and Griseo respectively)

Despite their accomplishments, both of them are insecure as they question their methods of handling situation and would ultimately continue to lose people in their lives be it separation or death and have their minds filled with nothing but regrets as a result of the choices they made

Both would participate in many large scale battles (Keyblade Wars and Honkai Eruptions respectively) and at some point became all alone and find themselves lost in a way after sacrificing themselves for their friends against their fallen (Aqua fell into the Realm of Darkness after saving a possessed Terra while Fu Hua burned up the last of her sentience against Kevin and fell into a slumber)

Both would end up having their bodies taken by dark counterparts of themselves (Phantom Aqua and Herrscher of Sentience respectively) as a result of actions of major villains (Ansem blasted Aqua with darkness while Otto Apocalypse shot Fu Hua and attempted to repair her body only to make a Herrscher instead)

Both of them would eventually be rescued and saved by two optimistic main protagonists (Sora and Kiana Kaslana, respectively) and returned to the light

Both would eventually join the heroes in major wars and fight against villain organizations (Organization XIII and World Serpent respectively) led by the series main antagonists who are fallen heroes (Master Xehanort and Kevin Kaslana respectively)

Additionally both are actually older despite their teenage appearances (Aqua is 28 years old but remained 18 despite being in the Realm of Darkness for 10 years, while Fu Hua is over 50,000 years old but remained 17 due to being a MANTIS Solder for some time before being mortal again)