I think, if forced to do so, ISTJs would be able to discard methods that didn't work. The situations Heero was in were frequently life-threatening, being part of the Gundam series and all.
Kingdom Hearts I can do.
Sora: People say ENFP, and I agree. Helping is his thing.
Kairi: ExFJ. I might say ESFJ, but she's not a very realistic character - her actions contradict themselves.
Riku: I've seen him pegged as INTJ, actually, but he seems ISTP.
Supposedly the Nobody characters have no hearts, but the trio that supposedly do (Axel, Roxas, and Namine) can be pegged.
Namine: Seems INFJ.
Axel: He seems ENFP, although under-developed; supposedly, darker ENFPs manipulate to their advantage.
Roxas: Unrealistic character, really. I think he seems INTJ in the Organization, ISTP after, with and without his memory. Overall, INTJ.
The other Organization characters aren't really meant to be typed, but I'll try.
Xemnas: ESTJ? Perhaps ENTJ.
Vexen: I've seen him made out to be INTJ, but I think ISTJ; he's concerned with the hierarchy of the organization.
Zexion: Stereotypical INTJ, down to insulting people for fun.
Axel: See before.
Marluxia: xSFJ? You don't ever see much of his character.
Larxene: Wonderful character; ESTJ.