Shelito Berylmarine is the son of the Berylmarine family, and governs over the Gemstone Kingdom as one of its statesmen. He is also an accomplished sorcerer, and is the older brother to Aqua, though that's not exactly something he takes pride in. He is best known by his alternate name, Paz.
As a statesman, his ambition is simply to rule the kingdom to his liking, though having the power to rule over the entire world wouldn't hurt. He collects the gemstones just like every other playable character, but not to use their power; instead, he wants to seal them away so no one else may use them.
As a sorcerer, he wants magic to be the reigning power in the Gemstone Kingdom, so he wishes to abolish science, mystical macguffins and any other means of power there is to compete against. However, he does use the gemstones in the PlayStation Original Mode endings, though one can assume that's non-canon and merely a "what if" scenario.
Paz has the unflattering role of serving as the game's villain, in a sense. He has crossed a couple of people the wrong way; he taught Sapphire sorcrery, who subsequently used it to terrorise her family; his slightly-tyrannical governing has rubbed Garnet the wrong way; and Rald doesn't like him purely because he's a magic advocate.
However, his quest is simply to increase magic awareness and to rule things the way he likes - he holds no personal agenda against anyone, but it just so happens that people get miffed over what he does. Even for characters whom he's had no real interaction with (like Pearl, for instance), he's still placed as the final character they face in their stories. One can assume an ambitious and powerful (both politically and magically) authority figure just makes a good go-to guy for quick villain material.
Paz is so invested in his goals that he has little time to make friends or allies, or even just socialise. His sister Aqua respects him, though he tolerates her at best; the two are such polar opposites that any chance of decent interaction is minimal.
He respects Sapphire as a student, though he expects her to unendingly obey him as her master, namely so he can take her gemstone. Sapphire holds enormous respect for Paz, and even great affection for him; her GBC ending shows the two expressing those thoughts quite clearly to each other.
Other characters have strong opinions of Paz, though he doesn't feel the same in return. Opa either admires Paz as a sorcerer statesman or has the hots for him (see Opa's page for details!). Garnet treats Paz as a villain to be kicked out of town, though Paz just sees him as a kid and doesn't give him any consideration. Rald disapproves of magic as a whole, and thus bears a great grudge against the sorcerer. By isn't too chuffed about him teaching her daughter magic as well, though Paz doesn't care one whit.
Shelito "Paz" Berylmarine
Paz resides in the "State", a castle on a floating island (which is seen in nearly all the backdrops in the game). If there's a proper name for the building or the island, it certainly isn't said in the dialogue; I had to glean this name from the arcade map.
If there's meant to be any theme to Paz's real name, "Shelito," (or "Cielito", an alternate translation), it's certainly alluded me.
His family surname, Berirumarine, can also be translated as "Berylmarine", tying in with how the topaz gemstone is related to the beryl mineral.
Paz tells you to RTFM.
Paz steals everybody's gems and doesn't even bother to use them. The cheek!
Paz decides to use the gems after all.
Paz tries to eliminate the competition by sealing away the gems.
Paz's attack pattern is to fill the left four rows, in contrast to Sapphire. This is a very strong pattern, dropping dense layers of blocks on the opponent's side; a good combo can be an easy victory. It's a very all-or-nothing tactic, however; either you wipe out the opponent quickly, or mildly hinder them.
When facing Paz as an opponent, reacting is the key - chip away from the inside or outside, but you need to do it fast before a combo can demolish you.