"This planet [Nemesis] is like my brother. He's a ruiner. His plans always went wrong. He put the Evil Black Crystal into the Earth, and made it a planet of death. That wasn't part of the plan. I'd like to let you go..." - Safire to Usagi, Act 20
The first thing you notice about Safire is that he looks amazingly like Mamoru. He's tall and lanky, with short black hair and the same long eyes. Upon closer inspection, though, you'll notice that Safire's hair is slightly flatter and tends to dangle in long strands over his eyes. Mamoru-kun's hair is swept back a little more, and has a more distinctive part. Still, the resemblance is uncanny.
Safire wears a black uniform with dangly white things on the chest, black gloves, and long black pants. Like the others, he wears two diamond-shaped black earrings, and his hair covers the upside-down black crescent on his forehead. His name is derived from the gemstone sapphire. The spelling of his name is debatable; the katakana are given as "Sa-fi-i-ru." In English, this could become anything from "Safir" to "Sapphire" to "Safiru;" I just picked "Safire" because it appealed to my personal taste.
Safire is the alchemist of the group. He works closely with the Evil Black Crystal, and creates droids for the Black Moon powers to use against the senshi. He is the cautious one of the group; he's not as power-hungry as the rest of them and quite content to sit tight with what he's got. He was also the only one to regret the loss of the Ayakashi sisters; admittedly, he didn't miss them as people, but he regretted the loss of useful soldiers. He always advises caution, but no one listens - especially not Diamaund, who thinks that Safire's after his power.
Safire was the first to suspect that things on Nemesis weren't as they seemed. He tried to voice his suspicions to the others, and got ignored and rebuked for his troubles. In fact, Diamaund thought that Safire was trying to steal his authority, and retaliated. (When Safire reproached Diamaund, his white-haired brother made glass shatter near his face. A shard slashed Safire's cheek, and he quickly decided that silence was the wisest course of action. In the Mixx English version, apparently Diamaund slaps him. Same result.)
Safire's not immune to power; he wants it just as much as his brother does. He's just more careful about it. At first, he gloried in the sheer power of the Evil Black Crystal; however, he realized that it was growing out of control. Unfortunately, he was unable to convince anyone else of the fact. In desperation, he blamed the whole thing on Usagi. He cornered her and knocked her down. "It's your fault," he snarled at her. "It's because of you that my brother's been acting strange. It's your fault everything's gone wrong! The existence of you and your ginzuishou have upset history." Quite forgetting that it was he and his siblings who had upset history, he tightened one hand around her neck. With the other hand, he summoned a huge sharp black crystal. "Die!" he cried, and plunged it towards her.
This turned out to be a bad decision. Usagi transformed into Sailormoon, and began tearing the palace down around them. Safire was knocked unconscious; when he awakened, Wiseman took over his mind and gave him the power of the "death hands." Upon Wiseman's orders, Safire tried to kill his brother Diamaund. Diamaund promptly destroyed him. You've got to feel a little sorry for Safire. He died without even getting his own mind back.
Credit goes to Ian Miller, who informs me that the term "safi~ru" is the Japanese approximation of "safir," the Old French word for "sapphire."