Sapphire is Prince Diamond's younger brother (and quite possibly Petz's ex-boyfriend). He doesn't really get much screen time until after Emerald dies and even at that, he doesn't really get to be seen until the episode in which he dies. That seems to be when he gets the most character development. Until then, he's pretty much around in the background making Emerald's crytal statues, or making droids, or keeping an eye on the energy level of the Black Crystal. Although he bears the mark of the Black Moon on his forehead, he is the only one of the clan not to wear black crystal earrings. It is noted by Diamond in the manga that this is his one symbol of defiance: although he is loyal to Diamond and will do what he wants, he is cautious and does not completely trust the Wise Man (who is the clan's advisor and presumably the one who handed out the earrings).
The Sapphire story begins shortly after Rubeus dies. He agrees to make Emerald's little statuettes, and warns her against failing at her job like Rubeus did. He also tries to talk to Diamond on numerous occasions about how he feels regarding the Wise Man and the clan's motives. Sapphire becomes a bit disgusted with Diamond's crush on Sailor Moon and feels that at that point things are starting to get way too out of hand. Sapphire blames the Wise Man for leading Diamond astray in the first place, and decides to spy on him to see what he's really about. After hearing the Wise Man muse about silencing the universe in the name of the Doom Phantom, Saph decides to cut the power off to the Black Crystal by swiping its main control card. Unfortunately, unbeknownst to Sapphire, the Wise Man had become aware of being spied on and had followed him down to the control room to confront him and kill him. Sapphire manages to teleport out of the path of the large blast of energy meant to do him in and travels to 20th-century Tokyo to go find Diamond and tell him what he's discovered. However, Sapphire is injured and stumbles into a park, where he collapses and passes out next to a tree. He has a dream in which he and Diamond are kids, and they're standing over a flower talking about the lack of them on Nemesis when Diamond vows to invade the Earth so that they will have fields of flowers as far as the eye can see. Then the Wise Man shows up and Sapphire starts yelling at Diamond to get away from him and not pay attention to him. Suddenly, he wakes up to find himself in a strange bed.
The bed belongs to Petz/Prisma, who saw Sapphire stumbling around and brought him back to her apartment to tend to his wounds. He notes that she doesn't seem as hostile as she used to be and she explains to him why. This causes Sapphire to really question the clan's motives, and it becomes all the more imperative to him to get to Diamond and deliver his knowledge. He tries to climb out of bed and is uneasy on his feet. Petz begs him not to go, but he leaves anyway, vowing to come back to her when he's done. Sapphire makes it to Diamond, who thinks he's turned traitor, and starts to try to explain what he's found out when the Wise Man appears from behind Diamond and delivers another blow. As Saph is sputtering to get the rest of his words out, he is hit with a third blow, which kills him. However, in death he makes his point and begins to loosen the grip the Wise Man has on Diamond.
Like Emerald, Sapphire also has two personal servants that do his bidding, Veneti and Aquatiki (I think they may be female). Unlike Chiral and Achiral, it's clear that Saph's servants are near-perfect droids that he's created by his own hand. Unfortunately, Veneti and Aquatiki didn't make it into the anime, so you never get to see what they would've looked like ;~;.