Asriel has fairly low stats for a mid to late game costing card, however it can heal all ally monsters when placed, as well as removing all negative effects, such as KR, Paralyze and Silence, makin Asriel good against Perseverance and Patience.
If someone has this card, it's best to save it for late game along with your other better monsters. Asriel is good in most decks, especially Kindness and Determination. Also good to revert the damage done by cards like Woshua, Same Fate or Strafe and prevent an incoming Asgore. Asriel is also extremely good after the opponent has used Toriel, because it will return your other monsters' ATK back to normal, but keep their Taunt.