First off, I hate the notion of it either being 7 companions + sidekicks, or 7 companions. Surely, the time and resources invested in the 6 sidekicks could have made another companion or two.

A sidekick is a unique model and portrait, gameplay voice set, introductory conversation and a few other lines in sidequests.

 

A companion is a unique model and portrait, gameplay voice set, introductory conversation, personal sidequest with multiple outcomes, ambient banters, relationship system integration with the player and all other companions, multiple branches of dialogue based on likes and dislikes, reactive lines along the critical path, and ending slides based on quest, relationship and main quest ending choices, all of it fully voiced. Trading in the resources it took to make all the sidekicks might get one companion

Confirmed in the other thread that it is a lot of work indeed. Too bad Ydwin is unlikely to become a companion, but as AndraColombo said there, personal quests and relationship system are not the most important things, so if more content is coming for Ydwin, it is good news for me.