The traveler is unique among current playable characters in that they can change elements. This is done by activating different statues of the seven in different nations. (While Childe is shown to wield an electro delusion, he is only playable as a hydro character. And we are unable to play in delusion form.)
There is a theory that the traveler's abilities are similar to those of the archon of the corresponding element. The Anemo traveler has vacuum and cc abilities similar to Venti. The Geo traveler can create geo constructs like Morax. The Electro traveler can restore energy like Raiden. The Dendro traveler provides off-field dendro app like Nahida, and their burst gains different effects depending on whether electro, pyro, hydro are infused. Certain aspects of the traveler's kit are indeed similar to the archons, but there are also differences.
Also this theory was basically debunked with the Hydro Traveler's kit, which is more similar to the Water Dragon's kit (with the droplets) than to the hydro archon Focalors.
At the start the traveler and their twin sibling fight some unknown god. You choose which sibling to play as (male or female). The traveler is then encased in cubes from the unknown god and gets separated from their sibling. Some time later the traveler wakes up in Teyvat and fishes Paimon out of the coast of Mondstadt (facing towards the time island). The traveler and Paimon then travel through Teyvat's nations in search of their sibling. Along the way they meet Dainsleif, a mysterious figure from Khaenriah. The traveler learns that their sibling had witnessed the destruction of Khaenriah around 500 years ago and had since then become the leader of the Abyss.
In the Sumeru chapter, while saving Nahida and defeating Dottore and Scaramouche, the traveler learns that they are the 4th 'descender' of the world (the Heavenly Principles being the first). Moreover, their sibling is somehow not considered a descender, and the ley lines perhaps recorded their sibling as being part of Teyvat. The traveler continues their journey, determined that they will one day be reunited.
The traveler does have specialized use in various teams. The c6 anemo traveler can be used in Xiao teams to shred -20% anemo res through the q. The geo traveler can be used in Itto mono geo teams (geo traveler q can buff crit rate and provide sub-dps). The electro traveler can be used in Eula superconduct teams if one does not have the Raiden Shogun. The dendro traveler can be used in dendro national teams and Nilou bloom teams.
The anemo traveler also has cc abilities that can push away enemies from monoliths in certain spiral abyss challenges. The geo traveler's meteorite can also block archer arrows from hitting the monolith.
Do ATK-dmg bonus-CRIT.
For dendro traveler (DMC), one can use gilded or deepwood. The DMC has an EM to dmg bonus % conversion, and gilded can give up to 230 EM. If one already has another teammate using deepwood (like Nahida, Yaoyao), then using gilded on DMC is much better than deepwood.
The DMC can also use full EM build for say Nilou teams.
The female traveler has higher charged attack scaling than the male traveler.