I just made a little search... Imakuni... it seems to be a NPC from the game boy TCG game. And also... a guy in a mouse costume...that, hum, ramdomly appears in some cards.
Heck It's the first time I hear about this thing. Somethings that I found:
"shining imakuni? is being given out to fun club members, and judging from previous fun club promos, this one too will be highly sought after."
http://www.sugarshock.net/zeo/main_archive/january2002.html
And you may want to translate this:
http://www.nintendo.co.jp/nom/0001/anive/page02.html
(my computer doesn't recognize the japanese characters)
And look at this:
http://members.fortunecity.es/pokecartas/1imakuniCD.jpg
"Imakuni?
A very strange man, Imakuni? is a mystery even to the other people in the Clubs. He claims to be a musician, but also wears black mouse ears on his head, so who knows for sure? He will appear randomly from time to time in a Club, and you can challenge him to a duel if you can find him.
Imakuni?
Prizes: 6
Win: One of each booster
Imakuni? uses a Water/Psychic type deck. A decent Lightning or Psychic deck should be good enough to beat him, especially since he insists on playing his trademark card. The Imakuni? Trainer card is an interesting collectible, but all it does is Confuse your own active Pokemon. Imakuni? will use it all the time, and if you beat him enough times, he'll give you one of your very own!"
http://guidesarchive.ign.com/guides/11879/side.html
Also, you will find some interesting images google searching.
Ask more about this at the TCG section, they probably know more about this.
Hope this helps ^^.