The story is after a RESET. Each part is a different timeline or route. The begginning of each route is explained after the events of before RESETTING the timeline.
Will be focussed on Genocide and Pacifist, Neutral route will be skipped as lot of different possibilities can happen, with endless of different outcomes and consecuences.
Neutral routes will be explainned if I see interest, ask and comment your ideas of what your choice in some parts of the story should be, and I'll do parts explainning this choices, and the outcomes.
Genocide starts after Frisk sells it's soul to Chara, and after RESET, visually, was still Frisk, but deep the body, the soul was Chara. Reflections of glass and water will show the reflection of Chara. Some characters will perceive Frisk not by visual, but other ways, and will recognize Chara, as the one in control; while others will just have feelings of a demon moving the strings. Chara realizes this route for simple joy, as see monster die, dusting in place, and the dust float to it's body to be absorbed as Exp, and the soul obtain more Lv, gives Chara a sense of power and control that overwhelmes them mentally, blinding them for power and have full control, while they take a blindless ragefull revenge in hatred against everything.
Pacifist starts after freeing the monsters from the underground of Mt. Ebott, and destroying the magic barrier spell. Frisk RESETS to can pass more time with it's friends, with desires of pass the eternity with it's friends, enjoy with them despite always be the same, see their happyness and happy ending happen over and over again, not grow up never and keep of an endless loop with it's friends forever. No matter how many times everything happens at the same, and will always be the same, the joy of monsters when are free, and their hopes rise as their dreams seems to become true, feels Frisk with DETERMINATION, as Frisk also becomes addicted to this feelings, making them have their so awaited happy ending, just to RESET everything and destroy that and replay it again, playing as their angel saviour over and over again. Despite be a pacifist route, is not truly pacifist, as the addiction of Frisk to domain monsters' feelings, hopes and dreams, and play with them, to just give itself a self pleasure of joy and happyness with this role, is what really makes Frisk be determined for continue, for more, and continue see their expressions be marked in their faces, their hopes rise higher with more light, and their dreams become true.
Genocide is the easy path, while Pacifist is hard mode, Chara will have a dofferent way on the story, as Frisk will have; some characters are more loyal to Chara than to Frisk, by different reasons, and also, the frienship will be different, as Frisk will have easier to make friends and allies, and obtain friend points with a character, than Chara, that will simply ignore this friend point in almost every case. Both routes have the same base story, but both have completely different events and outcomes, as in Pacifist, some characters will be friendly, but in Genocide, those same characters will act different, and may not be allies.
Every decision has its consecuences, characters will always remember; and who can be an ally, can become an enemy, and vice versa.
Characters beneficts:
Chara: Has much more dmg, def, speed, hp, and stats; able to use dash and determination slashes; has dominance over Asriel characters; it's determination at Lv20 is incomparable and will be the only one in control; better, easier, and full access to the save stars, being able to save and load in other timelines, in most of them with requirements to be met first; can learn magic; can use determination for offensive and defensive.
Frisk: Has more plotarmor; can call for help, receiving the help and assistance of others, including the human souls; has pure determination, making Frisk an undying version of theirdelves, as revives every time they die, but has requirements to meet; their determination can be absorbed by ally monsters for overcome challenges, and the monsters will accept to reset for bring Frisk back; Frisk has more easier for befriend monsters, and recruit allies; can save and load, but down requirements to be met; the human souls will assist and help Frisk, pureness of assist in their mercifull way to act.
Characters:
Enemies: Error, X-Gaster, GG!Underfell Sans, Betty Noire.
Neutral: Error 404 (William), Fatal Error, Ink, King Multiverse, Cross, Underfell Sans (Red), Noelle, Kris, Undertale Sans (Classic), Outertale Sans, Underswap Papyrus.
Ally: Core Frisk, Dream, Underswap Sans (Blueberry), GG!Underfell Flowey, Ralsei, Glitchtale Sans, Glitchtale Papyrus, Glitchtale Asriel, Glitchtale Chara, Glitchtale Frisk, Glitchtale Gaster.
Hostile: Dust, Horror, Killer, Nightmare, Shattered Dream, Susie, UltimateTale!Sans {'UltSans'} (Raziel).
The 'Enemies' are the main bosses to defeat for continue the trama.
The 'Allies' are the allies Frisk will have in pacifist route.
The 'Hostile' are entities who will be directly offensive, and can become either enemies or allies, depending the route and choices. Some characters will automatically choose path by theirselves, depending the route, as for example, Nightmare will be more 'gentle' in Genocide, because the negativity from Chara, and will be more aggresive in Pacifist, as the positivity from Frisk.
'Neutral' characters are the ones who can either offer some assistance, help in some moment, or anything else; they aren't hostile, but all depends of the route and choices, for example, Fatal Error is Neutral with Frism as the pacifist actitude, while with Chara will be hostile, and in any choice of Neutral route, if Frisk accidentally provocates Fatal Error, he will become hostile against Frisk.
I didn't add more characters in the list as ran out of ideas for who to add, but the multiverse is endless, so more will appear in the way.
Withouth delaying this more, and with all explained, enjoy this story.