- HENRY & SUZIE
I think that this one doesn't need an explanation! If there wasn't for Suzie, Henry could be with anyone else (that's not already in a group). But no, we can't put the brother and the sister to someone else. Not these two. Maybe when Suzie's older but not now! Henry was ALWAYS somewhere near Suzie when she became a Tamer. That's logical, he had to look after her. And Suzie never said: ''I want to go there with Rika (or Juri or Kazu or anybody)!!'' It's her older brother, they belong in a group!
- KAZU & KENTA
Best friends that are like alwaaaays together? They make the third group. You wouldn't put Kazu to Takato or Kenta to Juri, even though they're friends. They always stay together, we never saw them separated! Remember the episodes with Babamon and Jijimon or episode with Ryo or any other episode! They were always together! They even took a bath together at Babamon's house! xD Would you really put them in some other group?? No!!
- AI & MAKO
Ok, I won't even discuss this one. xD They had no relationship to anyone else but Impmon. Plus they're brother and sister. Again, in this age you put brothers and sisters together (for example Matt and T.K. or Tai and Kari in season 1!! Baby T.K. and Kari! Season 2 is another story ...).
So who's left? Alice? Yeah, ok, but you wouldn't put her to Rika or Ryo. No, RIKA and RYO are the two ones left in this story. It's not like in season 1, where almost nobody really had someone else to make a group, but in this case ONLY Rika and Ryo are left!! And they're not just two random people (as if they were Mimi and Cody or sth), they are two people that could have their own relationship! Takato and Juri are 'lovers' (xD), Henry and Suzie are family, Kazu and Kenta are best friends, Ai and Mako are family too. So what are Rika and Ryo? Enemies? I don't think so!
The picture in the layout is taken from a Digimon CD cover. Do you see that hair in front? It's Kazu! And guess what? Kenta is NOT on the picture, Kenta is posing with TAKATO and JURI - our first team on another cover! There are three different CD covers. One with Henry and Suzie (+ Yamaki, Henry's father) the second one is with Takato and Juri (+ Kenta) and the third one is with RIKA and RYO (+ Kazu). Kazu and Kenta, one of the strongest groups, was separated so that Rika and Ryo could be together on the same cover! And why is that? Because it would be odd to see Kazu and Kenta on a cover with Rika in the background or Takato and Juri with Ryo in the background doing whatever he would be doing. See?? Rika and Ryo are the only two left, that don't make a team to anyone else but themselves.

(Thanks to Chibimon Chronicle for the cover pictures!!)
