In KH2, when Sora enters Final Form, he is unknowingly channelling Roxas.
Evidence:
Final Form's color scheme is black and white, much like Roxas' outfit.
In official artwork, Final Form is often depicted wielding Oathkeeper and Oblivion, Roxas' weapons of choice.
Final Form is levelled up by fighting Nobodies, including Organization XIII. After losing Xion, Roxas wanted to take the Organization down.
Final Form only becomes available after Roxas and Sora's clash, which ends with Roxas fully accepting that the two of them are one.
Note- I have not played KH3 yet, so please do not bring up that game if you want to comment or add on to this. No spoilers!
