From Wikipedia:
Dragon Quest Monsters: Joker (ドラゴンクエストモンスターズ ジョーカー Doragon Kuesuto Monsutāzu Jōkā) is a Nintendo DS role-playing game published by Square Enix, and the fourth installment of the Dragon Quest Monsters series. Square Enix released the game on December 28, 2006 in Japan and in North America on November 6, 2007.
This is the first game in the series to have online play, via Nintendo Wi-Fi. Like the other games in the series, the character and monster designs are credited to long-time Dragon Quest series artist, Akira Toriyama, with the music composed by Kōichi Sugiyama.
The player's avatar is that of a tanned, grey/spikey haired youth with an editable name. He dreams of becoming a world famous monsters 'scout', which is a tamer of wild monsters who uses them in battle. There is a link to real world scouts in that there is a landmark in-game known as Baden's bell which is a reference to the scouts creator Baden Powell.
The battle system itself is very similar to previous Dragon Quest Monsters games. The player controls up to three monsters that make up the party, and can issue them direct orders or set them to one of 5 AI settings. The main character does not directly participate in battles except for when the player uses items.