Diamond is the stooge of the comedy duo, with his childhood friend Pearl playing the straight man. Despite appearing seemingly dopey and slow-witted when performing his role, he is actually very perceptive. His name is often shortened to Dia, but is also called by his full name, Diamond, by Pearl when the latter is unusually serious.

Diamond is often apathetic and rarely not seen eating - even in dire situations, as was the case when he had a feast with his Pokémon while aboard a Ginga-dan airship that was headed straight to their HQ. He shares these traits with his Pokémon, especially so with his Gonbe. He is much less active and energetic when compared to Pearl or Platinum, as seen when Platinum mused that every morning since the trio began traveling together, Diamond would perpetually end up being the last to get up. However, he is always serious when it comes to food and is an expert at cooking Poffins. He also displays an ability to analyze the situation at hand - when something strikes him as illogical, he will attempt to make sense out of it. Diamond has a somewhat natural instinct to judge how a Pokémon feels and can be quite observant when something arouses his interest. He is driven by emotion, thus his association with Emrit. 

~!For his skills, he is given the title of "Empathizer".!~

(Source: Bulbapedia)

Red is just a normal kid living in the rural Pallet Town, when he decides to go out on his own adventure, along with his rival since childhood, Green. In this world of Pokemon, he makes many friends, humans and Pokemon alike. However, all is not well. Team Rocket is trying to capture Mew, a very rare Pokemon, and is performing experiments on other Pokemon, trying to increase their power. Red and his friends must battle against Team Rocket to stop their cruel experiments and unlock the secrets of Pokemon.

Pokémon Trainers Pearl and Diamond are starstruck! Their lifelong dream is to floor an audience with their Pokémon comedy act. So how do they wind up as bodyguards to a pampered little rich girl on a quest to reach the peak of Mt. Coronet...?!
 
Come along and join the thrills and laughs as Pokémon Adventures continues!
 
Plus, meet Starly, Bidoof, Luxio, Combee, Bronzor, Rotom and many more Sinnoh Pokémon friends!
 
(Source: Viz Media)
 
In the Viz Media release, the Diamond & Pearl arc’s volumes were collected under the same title as the Platinum arc; this entry is exclusively for the Diamond & Pearl arc.

Finally together again, Dia, Pearl, and Platinum must battle Legendary Pokémon Dialga and Palkia, who are being controlled by Cyrus, the powerful boss of Team Galactic! Then a new Legendary Pokémon leaps into the fray! Whose side is it on...?
 
And who will rescue Platinum’s hapless bodyguards from the mysterious Distortion World they’ve been banished to...?
 
Plus, hang with Lopunny and meet Porygon-Z, Gallade, and... Looker?!
 
(Source: Viz Media)
 
In the Viz Media release, the Diamond & Pearl arc’s volumes were collected under the same title as the Platinum arc; this entry is exclusively for the Platinum arc.

A Special Music Video in tribute to the Pokémon Series of Games, set to the song Acacia by Bump of Chicken.

A story of a child's dream of the world of Pokémon. While his father works, the son explores the worlds of Pokémon throught he father's old collection of Game Boy, Game Boy Adance, Nintendo DS and Nintendo 3DS games and, as he does so, builds a bond with his father

Music video directed by Kawasemi for the song After Epochs by sasakure.UK feat. Hatsune Miku. This song and corresponding music video is apart of "Project VOLTAGE", a collaborative project between Pokémon and Hatsune Miku.