The Achievement Maniac is a rather old person who is still young in his/her soul. His/her dream was not to rescue the tuxemon world, or to become a good trainer, or anything like that... Instead, s/he wanted to do things that others haven't done.
At his/her advanced age, the Achievement Maniac cannot fulfill all wishes s/he had. All that's left are memories, and a Book of Wishes that s/he hasn't had a chance to complete. The Achievement Maniac is giving away his/her book of wishes ... Whenever you fulfill a wish, you can contact the Achievement Maniac and s/he gives you something for doing that. Because ... s/he likes you that much.
If you have achieved everything/achieved X number of things, he says:
"You have achieved so many things! But there is one thing I may yet achieve: beating a fellow Achievement Maniac in a tuxemon battle!"
After: "Hah! I have achieved something else: losing to a fellow Achievement Maniac!"
josepharaoh99 led the thinking about achievements/challenges. Tamashihoshi came up with the Achievement Maniac as a solution.