Obsidian

"Garnet: We need to be bigger. Steven: Then let's fuse, all four of us! It's the only way we're gonna get up to her head! Amethyst: I'm in! Pearl: I'm in, too. Garnet: Then it's decided. It's time to form Obsidian."

Obsidian is the fusion of Garnet (by extension, Ruby and Sapphire), Amethyst, Pearl, and Steven Universe. They formed for the very first time in their debut episode, "Change Your Mind".

Appearance
"They're huge!" Obsidian is a massive fusion with eight arms, glowing orange eyes, gray skin, and charcoal-black hair that reaches their knees. The skin and hair have bright orange markings. They also flaunt a mask (that features a nose similar to that of Pearl), which opens up to a third mouth. The third mouth is filled with lava that Obsidian uses to forge their sword. Their appearance greatly resembles that of the Crystal Temple's statue figure.

Personality
Obsidian has yet to show a distinct personality, as they have been shown to be very silent, apart from making vocalizations such as growls and roars, indicating that they possess a ferocious side.

"Change Your Mind"
In an effort to ascend the complete Diamond Mech, Steven suggests that the four core members of the Crystal Gems should fuse. The other Gems agree, and Garnet declares that it is time to form Obsidian. Garnet, Steven, Pearl, and Amethyst soon perform a fusion dance before coming together to form Obsidian. As Obsidian lets out a tumultuous roar and the other Crystal Gems express their awe in seeing Obsidian, the Diamond Mech soon returns. Obsidian begins to climb up the mech briskly while Connie, Bismuth, Peridot and Lapis distract the upper part of the mech.

After ascending most of the mech, Obsidian begins to summon their weapon, a gigantic greatsword. Doing so leads to the attack of the Diamond Mech, managing to slice the wrist of the mech's right arm, and in half for the left arm. It then proceeds to immediately thrust the sword into the mech's chest, and Obsidian then leaps onto the the mech's face, roaring into its eye. However, the mech retaliates by smashing its face into an adjacent pillar, causing Obsidian to defuse.

Abilities
Obsidian possesses standard Gem abilities and traits. As the fusion of all four (five, counting Ruby and Sapphire separately) core members of the Crystal Gems, Obsidian is presumably the strongest fusion so far in the series, possibly stronger than both Malachite and Alexandrite.

Skillset

 * Greatsword Proficiency: Obsidian forms the hilt of their sword by combining Pearl's spear, Steven's shield, Garnet's gauntlet, and Amethyst's whip; then, Obsidian smelts the blade of the sword by opening up a third mouth full of lava and sticking the hilt into their mouth. The red-hot sword that they pull out of their mouth is then capable of slicing through nearly anything, as it destroys Yellow Diamond and Blue Diamond's ships with seeming ease. The sword has a series of runes that run up the length of the blade, though the meaning of these runes are unknown.

Unique Abilities

 * Pyrokinesis: When Obsidian was eventually formed, their eyes (and in their mask) ignited fire.
 * Lava Generation: Obsidian's third mouth acts as a forge and is filled with lava, which they use to craft their greatsword.

Connie Maheswaran
"Thanks!"

Just like the her components, especially Steven, Obsidian cares about Connie's well being. This is shown when they save Connie from falling to her death.

Trivia

 * The fact that Obsidian has the ability to create/hold lava within their body may be a reference to the fact that, in real life, obsidian is a type of volcanic glass formed when felsic lava cools.
 * One of the uses of obsidian in human history is as material for cutting and piercing tools. This is referenced through Obsidian's weapon of choice in the form of a sword, its extreme sharpness mirroring the real-life obsidian's own.
 * Obsidian blades were the primary material components of Aztec weapons like the macuahuitl.
 * The name of Obsidian was first seen in the Steven Universe: Art & Origins book.
 * Obsidian's sword strongly resembles both the Flameblade and Great Flameblade  from The Legend of Zelda: Breath of the Wild. The sword's fiery properties and color resemble the Flameblade, the sword blade's outline more resembles the Great Frostblade, and all three blades have geometric patterns on the blade surface.
 * Obsidian has been formed before but with Rose. The Obsidian fusion with Rose is the basis of the statue that makes up the Crystal Temple, and was the fusion that led William Dewey to the beach where he established Beach City.
 * According to Joe Johnston's tumblr, Obsidian uses both she/her and they/them pronouns.
 * Obsidian also bears a resemblance to the Hindu war goddess Kali, who is commonly depicted as an ash or blue skinned warrior woman with many arms and long black hair. However, this is unconfirmed to be the design inspiration of Obsidian.
 * Currently, Obsidian is the first 5 cross-gem fusion seen in the show.
 * Due to having all of the main Crystal Gems weapons, Obsidian should also be able to summon Sugilite's flail, Opal's bow, Sardonyx's war hammer, Rainbow Quartz 2.0's parasol, Sunstone's suction cups, and Smoky Quartz's yo-yo.