Asriel and Chara Dreemurr
Their tale goes as told in classic Undertale, however, Chara requested that they’d be buried underground instead of on the surface.
Asgore finds the young heirs body, and takes them to the lab, where they’re resuscitated, and Chara’s soul splits from Asriel when he’s on the surface, reverting him to his child form.
Asriel is killed by the humans, and takes the final blow when he falls underground once again like the humans to come would.
Chara grows up with Asgore, who still executes his policy on humans, with Chara being the exception.
Toriel has no knowledge of Chara’s revival.
Chara cannot reset, or save, due to their determination being drained when Asriel absorbed their soul. This also means, that if chara really wanted to help break the barrier, and be the 7th human soul, it wouldn’t work, because their soul would shatter instantly after death, like any monsters would.