Characters will get stronger from fighting and training. Victory doesn't determine the gain in a fight but rather how much they exerted themselves. As most training will probably go by during between-arc timeskips or the like that will be a bit trickier. But as for the fights I will judge on a case-by-case basis to see how much the fighters get. I know it's a rather subjective method but making it too rigid will require putting a new system in place that will take some of the fun out of fights.