Diamante, often called Dia, is a gemstone character from the Land of the Lustrous series. They possess the highest level of hardness, rated at ten, which signifies their great offensive capabilities and formidable strength in battle. However, this high hardness is contrasted by their lower level of toughness, as their body is a single large crystal rather than an aggregate. This makes them paradoxically prone to cracking and shattering upon heavy impact, a physical vulnerability that is a source of deep-seated frustration for them.

On the surface, Diamond presents a personality that is gentle, kind, and compassionate. They are often perceived by others as elegant and serene, even in stressful situations like combat, and are considered one of the more approachable gems who will offer guidance to those in need. They have a well-known love for romance, often perceiving the interactions of others through a romantic lens and delighting in the idea of love stories, a trait that can sometimes amuse or annoy their peers. This caring nature drives them to be selfless and helpful, as seen when they relentlessly search for a way to help Phosphophyllite, believing them to have been transformed into a snail, persisting even when other gems had given up.

Beneath this amiable exterior, Diamond harbors significant insecurity and a strong sense of inadequacy. This internal conflict primarily stems from their relationship with their partner, Bort. Although Bort is also a diamond-type gem with the same hardness, their toughness is of the highest quality, making them a far more resilient and powerful fighter who can effortlessly withstand blows that would shatter Dia. Diamond is acutely aware of this disparity and feels inferior, frustrated that despite being the hardest, their body fails them. This feeling is exacerbated by the fact that Bort frequently has to save them in battle, to the point where Bort is said to no longer truly let Dia fight. Dia pushes themselves to the point of self-destruction, secretly developing new fighting techniques that put immense strain on their body and cause it to break, all in a desperate attempt to close the gap between them and Bort. While they admit to loving Bort, they also harbor darker feelings of resentment and have wished for Bort to disappear, a conflict that causes them great personal turmoil.

In the story, Diamond initially serves as a senior fighter and a figure of admiration, particularly for a young and useless Phos, who sees them as a perfect role model. Their primary role, however, is defined by their partnership with Bort, which is a central point of tension and development. As the narrative progresses, this partnership dissolves when Bort decides to team up with Phos instead. This decision leaves Diamond feeling abandoned, unnecessary, and forced into a position similar to the one Phos once occupied, which deepens their feelings of resentment and inadequacy. A key moment of individual development occurs when a powerful Lunarian creature attacks the school. With no one else to save them, Diamond is forced to confront their own insecurities. They choose to fight alone against the monster, and despite suffering catastrophic damage, losing their arms, legs, and parts of their face, they are able to defeat it, proving their own strength and resilience.

Notable abilities include their proficiency with a katana, and they are capable of incredible feats of agility, such as jumping great heights and running up vertical surfaces. Their most unique combat technique involves using their sword to deflect and redirect projectiles, like spears and arrows, fired by their enemies, the Lunarians, flinging them back with enough force to destroy them. This method, however, is extremely violent on their own body, often causing fractures as a direct result of the technique. Later in their development, Diamond discovers a talent for singing, which leads to a significant personality shift, turning them into an egoistic diva who craves attention and adoration, a stark contrast to their earlier, more humble personality.