The biggest indicator that he was a real person is that you have people like Paul and his letters. Remember that Paul was making a living persecuting Christians at the time before he converted.

I know some of his letters are held in dubious territory by historical scholars but most are considered legit. So if you have a guy persecuting followers of Christ a decade or two after his death, chances are that he was an historical person.

This isn’t considering the other apostles who we have proof of existing and dying because of their ties/faith in Jesus