What if in the Three Siblings AU, Chara tried to do the plan to break the barrier with Undyne instead of Asriel? How would that go?
Oh boy, that would go pretty different! One of the main points of the (tentative) canon for the AU is that the original plan didnāt go through, and Asriel and Chara were able to grow older and reach the point where they met Undyne. But! I think this possibility would be fun to figure out too! (And by fun I mean sad!!)
If Chara went directly to Undyne about it, sheād probably agree to it like Asriel did. But I think Undyne was more willful than Asriel as a kid, so when Chara started getting sick, thereās a good chance sheād bail and tell Asgore and Toriel. (Iām not sure if that would change Charaās fate, but I hope so!) If she didnāt bail, the next sort of divergence from the plan would happen in the human village.
Undyne still isnāt super on board with humanity in this AU, but she is a kid, and I donāt think sheād actually want to kill anyone went the time came. But sheād probably be fine with self defense and fleeing. Chara and Undyne would have an argument after they retreated to safety, and ultimately I think Undyne would win it, deciding they shouldnāt have gone through with the plan. Chara would be too worn out to change her mind once sheād decided this.
In the end, theyād head back to the Underground because they wouldnāt know what else to do, and both would survive. There would be a lot of stuff to work through in the aftermath of the planās failure, but eventually I think Undyne and Chara would be able to adjust to sharing a body. They donāt return to the surface (at least, not til theyāre older), because the responsibility of trying to free an entire kingdom is way too much for two children. So the events of Undertale would probably still come into play, just with a more peaceful Underground.
(Hereās a bonus drawing!)