Board Thread:General Discussion/@comment-30664268-20170429210906/@comment-13682699-20170503024648

He was SAVED in the sense that he was finally able to remember that he too was a good person before all that happened to him. He made some serious mistakes, and he deeply regrets it, but he is able to finally be at peace with himself, his actions, and what happened to him. With his temporarily recovered emotions, he sacrificed his ability to retain his form and used the comobined power of all the SOULS in the underground to break the barrier and free all monsters. During the few moments of being himself again, he was still a truly good and selfless person.

What happens after that is up to each and every individual person's interpretation. Technically Asriel turns back into a flower, but Undertale is very open ended, and there are many legitimate ways (even Toby himself has said a few were possible) that people have thought of that would allow him to either revert back into his true form or to simply retain his body shortly after the barrier breaks. One of the oldest and most popular is the Dreemurr Reborn series, which I highly recommend that you take a look at.