Beelzemon and... Yamaki?
Idk, this was really hard, this where a bunch of episode where a lot happen. you can pick Rika since here is when she completly opens to the group and her mother, a lot of development there. Takato, and Herny since we finally get a backstory of why he didnt want to fight. Alice, wich might be a ghost, does a lot here, heck even Makoto and Ai are great here. Even Ryo does something here, but you all hate him. and if you pick him would be the biggest trolling since the Deva's Owl.
I the Digimon departament you can choose Dobermon since he sacrifice himself to allow biomerge evolution, MarineAngemon does something here too.
but finally I choose Yamaki because here we see how guilty he feels about everything he did, and he does help the kids and the hackers. I just love this kind of redemtion character arcs. which is why I also pick Beelzemon. and Just picture this, Yamaki and Beelzemon both dressed in black. if Yamaki had had Beelzemon since the begginng it could have been a great villian. and they would have killed all the wild ones. and Riley might have got BeelStarmon because shipping reasons.