Chrono Trigger DS
The Shades are strange doppelgangers of Crono, Marle, and Lucca that lie at the end of each dimensional vortex entry point. Crono, Marle, and Lucca must each be present to progress far enough to see them. They exist in pale reproductions of the Millennial Fair, Guardia Castle, and Lucca's house. Armed with significant combat capability, they each battle Crono, Marle, and Lucca. The Crono Shade's raised weapon and the others' unused lines show that they are malevolent entitites:
[Marle Shade] One of me is all this world needs. Begone, for all eternity!
[Lucca Shade] You are all that stands in the way... You must die!
Defeating them awakens latent powers in the three. As part of the Dimensional Vortex and Dream's Epilogue ending, their canonicity is questionable. There are a few theories:
Memory or Frozen Flame Shade
At Terra Tower in Chrono Cross, the Frozen Flame shows darkness to whichever team members approach it. Guile (likely as an inside joke to the scrapped plan to make him Magus) sees a dark version of himself:
Guile: Is this what I have been searching for all this time... No... Stop it! I am no such thing. What good is it... to possess such dark power...?
The Shades are similar; they're dark memories or versions presented by the temporal chaos (since the Frozen Flame doesn't seem to be in the temporal vortex) to weaken the heroes or make them question their lives.
When fighting Crimson Shade (Dark Lucca)... the Eggsterminator that she had with her reminded me of the mini-robo that Lucca had in the ending movie when she finds the baby in the woods. Although they do not look all that alike, Eggsterminator might possibly be the "battle mode" of the mini-robo.
If this is so, then are the shade versions of Crono, Lucca, and Marle future versions of themselves in some dimension? In all 3 boss fights with the shades, it is always the shades that initiate the fight, meaning that they have some reason to want to fight Crono & co. Are future versions of Crono & co purposely trying to stop the present versions from venturing further into the dimensional vortex? Would Dalton have traveled to 1000 AD and raised the Porre army against Guardia anyways if he did not encounter (and was defeated once again by) Crono in the dimensional vortex? Does the defeat of these future versions of Crono & co have anything to do with their absence in Chrono Cross?
Chrono Cross Ghost Children
See Ghost Children, Shades theory.