(use vandusen)
(config `((host . "irc.freenode.org")
		  (nick "Koopain")
		  (debug #t)))

(define (youbi) (whisper-to "Youbi" "hello dear Youbi"))
(after-connect youbi)

;; Config is an alist, so you are setting nick to be ("Koopain") - i.e. a single-element list - and you have the same problem with debug.
;; It should read:

(config `((host . "irc.freenode.org")
          (nick . "Koopain")
	  (debug . #t)))