Arial is the third skeleton brother of the Gaster family, consisting of father W. D. Gaster, and Undertale sons Sans, Papyrus, and Arial. By extension, this means that Sans and Papyrus from Outertale have no father, and simply came into being. He fanologically is present in both Undertale (as the narrator only) and Outertale (as both the narrator and an NPC).
Arial is only slightly shorter than Papyrus, and has the most human-like skeleton out of all of the brothers. He is most often seen wearing a white-grey tuxedo with an unbuttoned dark grey blazer, slightly lighter pants with a black and yellow-buckle belt, dark grey shoes and a red tie. His face is seperated into five distinct areas: the forehead, 'eyebrows', eyes (which operate similarly to Sans'), nose-hole and mouth.
Upon Gaster’s dissolution, Sans received the ability to traverse time and Arial received the ability to traverse space. Due to the practicality of his powers, Arial’s induction as a space traveler came with a downside, a hollow SOUL, which meant he would need to lock-on to other souls in order to remain alive. He found that this need was easiest to fulfill by being a narrator of determined SOULs, and being around these SOULs for so long caused his SOUL to flip and turn red. Just like his name, Arial speaks in his respective font, but also in the default UI font when using telepathic communication. When fighting, all of his attacks consist of letters or symbols from the Arial font. While his SOUL is fueled (narrating someone), he can be defeated (reaching 0 HP results in extreme weakness), though not destroyed. Because Arial does not have superpowers concerning time, he merely exists on multiple timelines, rather than having connections between them, and thus is unable to reflect on previous RESETs. This does not stop him from commenting on RESETs before they happen, however.
Arial’s final form, wonderfully named ‘Final Arial’, is a failsafe for extreme cases only, for example, the PLAYER showing remorse for their actions after reaching the climax of a Genocide ending. When in his final form, Arial gathers the broken pieces of Gaster's soul, and with this power, creates a pocket dimension in which he and his victim(s) will remain until his mission is accomplished or he is defeated. It is difficult for Final Arial to narrate in this form, but he is able to at slowed and shaky pace. He will not be able to speak directly at all. In Outertale, it is impossible to land any attacks on Final Arial because of his absolute manipulation of space.