Bat!Chara (Vampire bat): Chara was the first human to fall in the Underground, escaping from her past life (her kind was disappearing and she was the last one who survived, the other human bats mistreated her and exiled her because of her behaviour). She was confused when she met some monster owls, but quickly Chara discovered that she was stronger than them and decided to take the power of the Underground, starting a genocide route. The King asked Gaster, his most loyal scientist and warrior, for help to stop her. (Some important facts: Asriel never met Chara, he was at the castle when she fell. The Dreemurr family just saw her from far away and never adopted her.)
Owl!Gaster (Sooty owl): He was the most important scientist and Royal Guard. He created a lot of magical weapons and studied the Barrier for years to find a way to get to the Surface. Everyone respected and loved him. When Asgore asked for his help, he faced Chara and did everything to stop her peacefully, but she was out of control. They fought for a long time with all their streght, part of the city was destroyed in the process. But finally, Gaster won and killed her! However, he was seriously injured and slowly dying. The last thing he did was to hide Chara’s Soul in a secret room from which only he knew about, so no one would ever find it and consume it because of the evilness and danger that it could cause. Asgore decided that every human bat that fell in the Underground again would be murdered, so this way he could protect his family and his Kingdom.
Years later, Bat!Frisk fell and (similar to the original Undertale story) she showed them that not every human is evil and helped them to go to the surface. Monsters had already six humans souls and needed hers to be free, but Frisk found Chara’s Soul and, avoiding to be corrupted by its madness and evilness, with it they opened the Barrier.
Bat!Frisk became their savior and friend. Monsters build a new owl city in the surface and a statue of Owl!Gaster, in the center, to remember him as a hero.
Trees ‘talk’ by exchanging chemicals.
They communicate through
underground fungi, and when they
can recognize their relatives, they
share nutrients. Basically, tree
'families’ help each other out. Source
There’s a huge ‘ocean’ hidden
underground, in the mantle of our
planet. Though it is not in liquid form
(or any recognizable form) and is
bound to the rock, it is likely our
largest water reservoir and suggests
that Earth’s water supply may have
come from within. SourceSource 2Source 3