I recently saw this one mentioned on Discord coincidentally. It’s signed by アクノウチ (Aknouichi), a “researcher” for Dark-Type Pokémon and assistant/coworker of Dr. Sameji.
Here is a Masaki (Bill) with the same signature at the bottom. The other two signatures are from Dr. Sameji and ハガネザワ (Haganezawa), the researcher for Steel-Type Pokémon.
All cards with the signatures of the Dark- and Steel-Type researches seem to come from the same event of December 19th, 1999.
All this information came from Emory on Discord (which I think is @dreamtech here on the forum?), so all credit goes to him.
EDIT: Also tagging @magicrap . Cool Pikachu card btw. I’m pretty jealous rn. 
Greetz,
Quuador