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!