He is an anthropomorphic goat monster who has long ears, a snout, visible fangs, is wearing black pants, and seems to be wearing a yellow and brown striped shirt with no sleeves due to the fact that he was so anxious to finish it, that he completely forgot to add sleeves. He also has a small wisp of fur, and appears to be lacking horns.
Asriel looks up to Alphys as a role model. He is an innocent monster who assumes an energetic attitude that pervades most of their actions. He seems to be a bit naive, as well as a bit clumsy, as he trips frequently.
Despite the protagonist notably frightening Asriel during the Genocide Route (once he discovers the protagonist's intentions, at least), he stand ups to the protagonist with an intense expression, ready to protect the Underground, trying to act like Alphys. Choosing to spare him and abort the route, however, reverts him back to a more favorable opinion of the protagonist, showing a rather forgiving nature.
Asriel first meets the protagonist in Sunnedout. He notes how both of them are wearing striped shirts, so they both must be children. However, he does not seem to notice that the protagonist is a human child.
He later sneaks out of Sunnedout to explore Firerise, looking for Alphys. He follows the protagonist, not knowing that the latter is a human whom Alphys is searching for, and he feels a little jealous of the attention that the protagonist receives.
Asriel eventually finds out that the protagonist is a human and reluctantly considers the protagonist an enemy. He tries to say something mean to the human, in an attempt to become enemies, but feel bad for it and start to leave.
Just as Asriel leaves, he trips over an edge and hangs from it while Alphys approaches. The protagonist must make a choice:
- Help him climb back up: Asriel defends the protagonist from Alphys, who backs off. He decides to remain friends with the protagonist and leave for home.
- Let him fall: Alphys dives down after Asriel to save him. She is badly hurt but refuses to rest. He then tells Alphys that she was right about the human, who just stood there waiting for them to fall. This response makes Asriel hate the protagonist and all humans.
- Run to Alphys: Alphys saves Asriel and is badly hurt. He tells Alphys that she was right about the human, who was too eager to fight her to bother saving them. This response also makes Asriel hate the protagonist and all humans.
- Run away from Alphys: Alphys helps Asriel up, who tells her that the protagonist was just going to get help from others.
Asriel is seen cheering the protagonist on to victory in the bottom-right corner as they face off against Temmie. In the epilogue, they can be found in Sunnedout. If saved earlier, he says he doesn't think Alphys is that great anymore, and he they found himself a new idol, likely to be Sans. He is also seen during the credits, attending Asgore's school. However, if the protagonist didn't helped him when he felt down, he will hate them forever, concludes that Asriel doesn't want to talk to them in the epilogue.
Despite being warned by Alphys that the protagonist has hurt a lot of people, Asriel approaches the protagonist in disbelief. The protagonist turns away, and as Asriel questions the protagonist about answering him, the protagonist silently approaches them with a "weird expression." Asriel begins to panic and wonder what he could do, and then stands up to the protagonist out of desperation, trying to act like Alphys, stating that if the protagonist wants to continue to hurt others, the protagonist would have to go through him first. The protagonist eagerly enters a battle with him. The narration then declares that the kid is "in [the protagonist's] way." After subsequent resets, the protagonist simply attempts to trick the child into turning the other way but fails and re-engages battle. The protagonist cannot escape this encounter.
- If the protagonist attacks Asriel, Alphys takes the blow in their place. The kid escapes while harboring a shocked expression, and the protagonist fights Alphys the Alpha. Asriel is not seen again for the rest of the game; it is assumed they evacuated with the others.
- If the protagonist spares Asriel, he says he knew the protagonist was not that bad and leave. This action aborts the Genocide Route, and the protagonist fights Alphys normally.
- Asriel is possibly born after the deaths of his two adopted siblings (Monster Kid and Frisk). Hence never mentioning them or know them. He also never mentions that Toriel is his mother or ever seen his father, since Asgore has left his throne before Asriel's birth.