Rex

Rex (, Rex) is the main protagonist of .

Origin
Rex is a 15-year-old salvager from who lives on a  named Azurda, who he refers to as "Gramps". He unwisely takes on a high-paying mission to help a group of mysterious individuals looking for specifically Leftherian help with searching for a great treasure. They succeed in dredging up an, aboard which is their target, a legendary caled the. Rex foolishly touches the red sword, so the group's leader immediately kills him, and the group's muscle  prepares to kill the rest of the salvagers to cover their tracks. Rex then finds himself in a dream of, the fabled paradise atop the he and many others want to visit, and meets with Pyra, the personification of the Aegis. She says they've been bonded, and she can revive him as her and thus wielder of the legendary sword, so she can return to the real Elysium. Rex agrees, and together with the defecting Nia and, escape the now-villainous group. In the process, Azurda is wounded, and manages to regenrate himself into a larval form that tags along inside Rex's helmet. Rex makes it his goal to bring Pyra to Elysium at any cost.

As the party travels, Rex encounters Jin and Malos several times, as they try to capture Pyra for their own reasons. This results in revealing Mythra, the original form of the Aegis, who created the alternate personality of Pyra in shame for her actions in the war five hundred years ago. Eventually, Rex connects with both Pyra and Mythra well enough that their trust produces a third form,, who is able to bestow Rex with additional "Master Driver" powers over Blades and the world at large.

In the course of his adventure, Rex becomes the figurehead of his party of paradise-seekers, each with their own reasons for following him and his optimistic goal. He starts out fairly naïve and all too confident in himself to solve every problem, but eventually learns that he can't do everything and must leave something up to others.

As a moveset element
Rex appears as part of and 's Final Smashes, where he aids them with their respective moves for the duration of the Final Smash; uniquely among assist characters in such a role, Rex's eyes will also glow when Pyra and Mythra have their Final Smash charged, as well as while assisting them in the attack. Rex also appears in their up taunts, on-screen appearance, and two of each of their victory poses. Unlike other taunts that spawn separate character models, Rex's model and animation in Pyra and Mythra's up taunts is completely independent of them, being unaffected by status changes such as Giant or Slow and persisting even when the taunt is canceled or they are attacked.

According to Sakurai, the development team considered having Rex fighting alongside Pyra and Mythra, but this was decided against due to the difficulty the team had with the in contexts such as 8-Player Smash. Sakurai specifically cited the far more complicated design and abilities of Pyra and Mythra compared to the relatively simple appearance and moveset of the Ice Climbers. Another issue was getting both characters to move at the same time which the development team found was impossible to do; similarly, it was deemed too difficult to implement Rex as the main fighter with Pyra behind him. This ultimately led to Rex being reduced to a smaller supportive role and Mythra being included along with Pyra with the two being able to swap instead.

As a costume
Rex was considered for the initial roster alongside other prospective characters like a character from, and 's ; however, he could not be added due to Xenoblade Chronicles 2 being announced after character planning finished in 2016. As an offered alternative, Rex appears as a costume for the. Azurda (otherwise known as Gramps) in his miniature companion form appears in the set's outfit. The costume comes as a free bonus with purchase of the Fighters Pass Vol. 1.

Other
Rex is seen in Pyra and Mythra's reveal trailer, searching for a seemingly missing Pyra. When he finds her on, she reveals that she was chosen for Smash over him, much to his surprise. He later goes on to provide commentary in awe of Pyra and Mythra's fighting prowess and eagerly hops in to conduct their Final Smash.

Spirits
Rex also appears as two different primary spirits.

In Adventure Mode: World of Light, Rex is presumably one of the countless spirits captured by Galeem during his takeover of the universe. After Galeem's defeat, Dharkon takes control of Galeem's spirits, including Rex. Rex's spirit is located in the Sacred Land sub-dungeon of The Dark Realm.

Rex uses a puppet fighter and is fought on the Kongo Falls stage's Ω form. If the player has already purchased the Pyra/Mythra Challenger Pack, Rex is fought on the Cloud Sea of Alrest stage's Ω form instead. He is also fought along a puppet fighter representing Pyra. During the battle, Shulk starts off with a Fire Bar and the floor is lava, referencing how Pyra is a Fire Blade with the ability to engulf the battlefield in flames. If the player has already purchased the Pyra/Mythra Challenger Pack then the Lava Floor hazard is not applicable.

Originally, the Rex spirit had no additional effects. However, with the release of update version, Rex's spirit can be enhanced at level 99 into his Master Driver form from later chapters of Xenoblade Chronicles 2. This is the first time a spirit in the base game was given the ability to enhance, or had their ability changed in the game's postlaunch.

Trivia

 * Like Creeper and Arsene, Rex has an internal codename. However, unlike those two, Rex essentially has two codenames, that being "diver" and "element". "diver" referencing his job as a salvager and "element" being a generic word most likely to describe Pyra and Mythra's powers, that being fire and light respectively.
 * Rex is referred to "element" in a folder containing his model, and is referred to as "diver" with his actual model. "element" is also the codename for Pyra & Mythra's victory theme, and the duo's codename when they are a pair.