Diddy Kong is Donkey Kong's buddy, nephew, and companion who has gone on countless adventures with him. Diddy is a creation of the British company, Rare, Ltd.; however, he has a Nintendo logo on his trademarked red hat, probably because Nintendo owned Rare and the rights to the Donkey Kong franchise at the time. The name 'Diddy' is a slang word used in some parts of northern England for 'small,' as Diddy is. Diddy has also gone on many adventures by himself and has even starred in his own racing game for the Nintendo 64 and later for the Nintendo DS.
Donkey Kong and Diddy Kong were enjoying the feast of the Mushroom Kingdom's Anniversary, when they come out (after some time Mario and the others get sucked in) they notice the Portal. They go towards it by curiosity and get sucked in. They later meet with the Heroes.