There are a few things that aren’t connected to any of the series, or rather are connected to all of them. I won’t care about spoilers, so you be careful.
I remember only fragments of the first season, Adventure… It was ages ago, watching it after school in german on RTL2. The kids were so many. Nine-ten? And they were so kids.
I guess it must’ve been hard for the writers to create a storyline where they could fit in all the characters, develop all of them well and still stay entertaining. Probably that’s why the two youngsters of Adventure come back in 02. In 02, the eight kids (counted, eight) of Adventure were made helping background appearances except for the two of them who became members of the new five person team. They were later joined by one more kid, a past enemy, going to six, the same pattern as in Frontier. In Tamers, in a totally different world, where Digimon is “just” a card game, the new team goes down to three kids, and they spend much more time in the real world, compared to the first two seasons or Frontier. In that aspect, Tamers and Savers are very similar. This number three seems to be the best one, because in Xross Wars the team is again of three kids.

