Diamond is the stooge (boke) of the comedy duo, with his childhood friend Pearlplaying the straight man. Despite appearing seemingly dopey and slow-witted when performing his role, he is actually very perceptive. His appearance is based on the male player character of Diamond and Pearl, Lucas. 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 Team Galactic airship that was headed straight to their HQ. He shares these traits with his Pokémon, especially so with his Munchlax. While not fat, 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 Mesprit. For his skills, he is given the title of "Empathizer" (感じる者Empathizer).

Diamond has a large amount of respect for Pearl and despite the latter's tendency to berate him aggressively, the two are as close as brothers. Diamond also has a not-so-small crush on Platinum, which he developed when they first met.

Diamond has changed a lot from the beginning of his journey with Pearl and Platinum. He was convinced by Pearl after the incident at Veilstone City to set aside their dream of becoming great comedians to protect Platinum until she finishes her journey. On Iron Island, he also received training from Riley in order to further his battling skills, which were lacking when compared to Pearl's and Platinum's.