He looks about the same as canon Undertale, except he has only one stripe on his shirt (much like canon Chara.) Also, he has different eyes.
At the end of the Genocide Route, Asriel behaves in a polite, deliberate manner, thanking the protagonist for guiding him. However, if the protagonist refuses to destroy the world, Asriel immediately becomes malevolent, despite the outward semblance of youth and innocence. Asriel reveals that the protagonist's "determination" and "human soul" awoke him from death as soon as the protagonist entered the Underground.