Flame on!! (Doesn't it kind of look like his head is on fire? XD) A lot of people seem to think Rubeus looks like a GQ model and with that pose he's striking over there, it's easy to see why. One of my friends once said that Rubeus was a jerk but that he had nice arms (heh). At any rate, Rubeus is the first to command a mission to kidnap/kill Chibi-Usa and get the Silver Crystal. The Four Sisters are his subordinates, but they too have subordinates of their own (the droids) that do most of the grunt work. Rubeus and the Four Sisters hang out in a UFO from which they watch the goings-on of 20th century Tokyo and plan their attacks.
The Rubeus story actually begins with Chibi-Usa falling out of the sky right into the middle of one of Usagi/Serena and Mamoru/Darien's hot dates. To make a long story short, Chibi brainwashes her way into Usagi's family with Luna-P, convincing them that she is a niece and/or cousin. She is there looking for Sailor Moon so that she can get Sailor Moon (or specifically, her silver crystal) to come help her mother in the 30th century. Unbeknownst to her (or maybe not), she was followed by Rubeus and his entourage, who are trying to alter the Tokyo of the future by messing up the Tokyo in the past. He sends the Four Sisters after Chibi to try and capture her, once they realize that she is after the Silver Crystal. The Four Sisters end up defecting, one by one, until Rubeus is the last one left. Emerald shows up and starts teasing him, and basically tells him that Prince Diamond isn't happy with his job performance and that he'd better work on redeeming himself. Rubeus comes up with a plan to bait the senshi, capture Chibi, and get the Silver Crystal. He captures the four planetary senshi first, then Sailor Moon and Chibi as well. Rubeus reveals a sadistic streak when he crucifies the Inner Senshi on crystal crosses and then shows them to Sailor Moon. He begins to physically torment her as well, by manipulating the gravity field on his ship (causing her to become weightless and then slamming her into the floor and crushing her). Meanwhile, Chibi-Usa has noticed that there is a certain crystal on board the ship that glows whenever Rubeus makes use of his power to manipulate gravity/fight Sailor Moon. She surmises that if she can somehow break or dislodge the crystal, then she can cut off Rubeus' power. Chibi goes to work trying to dislodge the crystal, despite the fact that there is a strong force field around it. She eventually succeeds, not only cutting off Rubeus' power, but also setting into motion a chain of events that cause his ship to begin to self-destruct. Thinking that the senshi are trapped in the ship with him (in high-Earth orbit), Rubeus declares that he would be willing to die if it meant that he was taking the senshi with him. Unfortunately, he wasn't counting on them being able to do a "Sailor Teleport". Emerald shows up soon after, to find Rubeus standing alone in the midst of a burning ship. Feeling relieved, he asks her to take him back with her (presumably to Nemesis) but she says no, to his surprise. Emerald teleports away just before Rubeus' ship explodes, vaporizing him.
In the anime Rubeus seems to have a 'thing' for Avery. It's mostly implied though, because no one ever comes out and says anything, but it shows in that Rubeus hardly ever criticizes or yells at Avery as compared to the other sisters. It doesn't mean, however, that he's 100% nice to her: In one episode he calls her a prima donna. But anyway... I know that in the manga he takes a liking to her and sort of favors her over the other sisters. There's a scene in there where he comes up behind her and kisses her on the neck. It's not certain if Avery is interested in getting serious or not; I know she has a bit of a crush on him because all of the sisters do. She returns the feeling, but I don't think it ever becomes anything major. Then there's Catsy. She swings the other way, in that Rubeus criticizes and yells at her the most out of everyone. I think he finds her highly annoying due to her eager-to-please willingness, which is a result of her severe infatuation with him. It's evident that he doesn't like her at all but instead takes advantage of her feelings for him by humoring her and therefore getting her to do a lot of the work.