(These are early backstories and designs. Will most likely be changed in the future! Never the less, enjoy the "dev-log!)
So.. Since we did a tie, we're doing both! Here we go:
The 1st sans-ational character is:
Sans!

Sans is the ruins dweller, as he explores with Undyne and has a very.. strange past, to say the least! He has common sense, ketchup, and a very high IQ to say puns. Impressive!
Why is he in the ruins to be exact? Well.. long long long-
Shut up and just show us gameplay!
Oh! Say hello to this person that likes to constantly be salty!
Anyway, long ago Papyrus & Sans were together, doing activities together, and everything all around.
..Until papyrus.. accidentally killed a human..
Sans was not afraid, in fact he was chill.
Can not be the same for papyrus though.
Papyrus got scared for his power, locked everyone out of his castle, including sans. sans went heartbroken, left the castle with Undyne as his gaurd..
They passed Toriel.. Toriel had stopped them, then asked them for a promise; take care of any human that falls down.
Sans & Undyne go into the Old Ruins, yet anybody can enter. It's a new paradise for anybody to be welcomed too.
Then CHARA fell. Sans, Chara & Undyne stayed in the Old Ruins for some time until another human fell down. The Kindness soul. Soon or later, Chara killed them. Chara took the soul, and for sans.. Sans tried to murder them. Sans was enraged, but chara seemed to escape this. Chara, in Snowstorm Forest, continued until Chara saw a very, very young Asriel. That is the end of the story for sans.
What about the other character?
Ah! Well:
The 2nd determined character is:
Chara!

Let's continue from sans' story!
Chara & Asriel grow up together and it's great!
..Since Chara killed a human and took it's soul, Asgore was fascinated. Chara was then took into the royal guard for training. The insane power from Chara was insane. Chara was soon then put into the royal guard and then, years and years later, killed 2 more humans. Many years later.. They have 6 souls. Chara has become a demon full of determination, can destroy anything without any empathy. Nothing can stop them. ..Unless it's Asriel. Asriel is the best brother to them and will try to stop them. Chara isn't causing a genocide is because of Asriel. Asriel is the main reason why no monsters are dead yet. Sans & Chara have a dark past, but Chara is willing to forgive him because of Chara's past actions for being so foolish. Then.. Chara became the captain of the Royal Guard. Asgore & Chara are bonding, and so is Asriel with Toriel.
Why does Chara look like that? It's creepy.
Like I said, it's an early design. Most likely will be changed later, but that'll be the end of this "dev-log"??
I hope you guys have enjoyed this.. And hopefully, we'll get more characters finished to show to you all.
Have a wonderful day!

-UTTSans