I think there is an argument to be made for Gaster Knight

Deltarune Discussion
1

(WARNING MASSIVE CH. 3 AND 4 SPOILERS)

the spoilers in question:

So Gaster is who we think is giving people shadow crystals, right?
Well what is the Knight’s sword made of?
Shadow Crystals.
Not to mention the hand holes and i guess the skeletal appearance.
Personally I still like Dess Knight more, but there is an argument for Gaster.
Or Dess could be serving Gaster and handing out crystals for him.
After all, a Knight needs to serve a king, right?

2

The knight's sword is made of Black Shards, an unrelated item. The Shadow Crystal drops out of their pocket or something. I should probably study the cutscene.

3

Symbol for it is literally a shadow crystal.

4

A small chip of extremely hard glass. Oddly, it's nearly opaque. - Black Shard Light World description

That Crystal... It's nearly invisible [...] - Seam, talking about the Shadow Crystal

Impenetrable by light; neither transparent nor translucent. - Opaque, American Heritage Dictionary

Also the sprite for the black shard is a different triangle. Although they do look similar and are both magic shards of glass, Shadow Crystals are see-through and colorless while Black Shards are black.

5

But both are dropped from the sword when Susie chips it

6

yeah but they're very clearly different items. When you give the shadow crystal to Seam you keep the Black Shard.