Ok so the reason why their name is Sarah, is because Kris has some of frisk's name (kris is just frisk but the k is swapped with the f and the f is removed) so I took that ideal and made a character with some of chara's name and they look like frisk, so I took chara and flipped it backwards and removed the C, and added an S so we have Sarah! and Sarah is a guy-

Sarah has kinda no emotional trauma or anything and in fact they are the exact opposite of Kris kinda, which means that Sarah legit hates going to the darkner world when he manages to fall down into there

Sarah however, has lost their right eye, because as a child, an accident happened with a monster who was playing with Sarah, as the monster tripped and clawed the eye out, Sarah was fine and he wanted to play with the monster again, but this led the parents of Sarah to despise monsters, but when Sarah gets into high school he tells his parents he wants to go to a different town for a better education, despite their judgement of not liking monsters, the parents went to an all monster school as it was the best school in the state.

The way Sarah goes into the darkner world, is by following Kris and Susie into an arcade as they were acting suspicious and found himself being alone in a darkner world, coming in last minute to save Kris, Susie, and Ralsei from the final battle, as Sarah has magic, and not act, which Sarah's magic, is extremely powerful, but also damages Sarah greatly, this magic, is self destruct magic, which is based on well. . .him exploding. He has 3 moves, each costing 25 tp, 50 tp, and 100 tp, his first move is a move that does 175 dmg, which makes Sarah take 30 dmg, (Sarah has 200hp because in the darkner world, he becomes a robot), his second move does 200 dmg and it deals 50 dmg to Sarah, and his third insta defeats ONE opponent but also leaves Sarah with 1 hp. Sarah can NOT kill people unless they use a 4th move in the genocide which using a special "Brass knuckle" (brass knuckles are his weapons) he can take NO dmg from any of his moves, and they do a crap ton of dmg, but this brass knuckle makes him take more dmg from other opponents.

In the end the final boss attacks the gang and Sarah comes in at the end and here are the 2 endings

pacifist: he comes in and uses his "final destruct" which is his third move, and he leaves himself at .01 hp, which makes him pass out, and leaves the boss with 1 hp as well, which allows the gang to stop the boss and destroy the well, but however ralsei can't heal non living objects (in the darkner world Sarah isn't a living thing, he's a robot) So Kris and Susei go to the lighter world and take him to the hospital which he stays there for the rest of the next 2 chapters (this would theoretically take place in chapter 5)

Genocide: With the "boom boom brass" weapon, Sarah uses his 4th move which kills the opponent as kris is one on one with the final boss, as susie and ralsei are passed out, Sarah goes and passes out because he feels bad for killing the boss. Kris destroys the dark well, and Sarah is now scared of Kris, and joins Noelle in finding what happened to Kris, and what he is really up to, as they realize the "dream" they had was not a dream

The character development Sarah has, is that Sarah, was never really LIVING a life, they were following everything his parents wanted him to do, he never had a choice in anything he did. And that's why he is visualized as a robot in the darkner world, because he isn't living. Sarah at the end of the pacifist, realizes that to become alive, he has to make some sacrifices and has to refuse commands from those who want to use him, Kris in the pacifist tells Sarah to go destroy the well, but it would have taken Sarah too long, and Sarah didn't want to let his friends die, so he jumps onto the boss and defies orders from the gang and realizes that he is now alive.

If you read all of this, you are a pog champ.