Wormmon brightens at this wonderful news. "Isn't that great, Ken-chan? I can stay with you."

"It is." A companion system....So that's why Wormmon was allowed to accompany him, when others don't necessarily have anyone from their worlds. It doesn't explain their missing Digivice, but there surely must be something Ken and Wormmon can do on their own.

Plus, Wormmon has an orange collar. He has a team affiliation, so...

"Do companions often help with missions?"