RosarioTale is an Undertale Au created by FanFiction.net user The SteelNerfer (yes, me.). It is a combination of the universes of Undertale and the anime world of Rosario + Vampire.
Warning, this contains spoilers for Undertale, Rosario + Vampire, and Rosario Vampire: The Revenant. I suggest you hold off reading this until you understand those stories.
The official story is called Rosario Vampire: The Revenant. The world is set in a post-genocide route where Chara/Frisk (who is a girl in this universe) is unwilling or unable to Reset. The story follows Dorian, a revenant who has been fighting Chara for two years before the events in the current story.
Dorian Grael is a revenant who was born in the same town Frisk grew up in.
For most of his current life he lived there and became friends with Frisk. He fell down Mt. Ebbot when he was twelve years old. He snuck out of the Ruins and avoided detection from Flowey or Toriel. On his way to Snowdin, he met Sans, who teleported him to the surface. Dorian asked Sans to teach him how to do that so he can visit him more often, so Sans does. The two become good friends. Sans teaches Dorian how to use Gaster Blaster and bones as well as how to use telekinesis and Karamic Reteboution (which Dorian calls poison). Two years before the events of Undertale, Dorian is forced to move to Japan because of his dad's job. He moves in across the street from Tsukune, the main protagonist of Rosario + Vampire. They become friends, before the two part ways, with Tsukune going to Yokai Academy and Dorian leaving to check on the Underground. After viewing tapes provided by Alphys, he swears revenge on Chara and watches the fight to try to figure out Chara's fighting style. While he's in the Underground, he notices that a human girl about his age has fallen down. He teleports to the Ruins and greets her, and the two become fast friends. After the girl joins Dorian on his quest, they leave through the now broken Barrier. During their travels, they fall in love and become boyfriend and girlfriend. During their last fight together, Dorian lets his guard down and is killed by Chara. In an attempt to save him, Dorian's girlfriend puts her soul into his body, saving his life but killing herself. Dorian then hunts down Chara, where he finds out that she's around Yokai Academy, and that's where the story begins.
Stats: HP: 1 (fast enough and has enough stamina to dodge far longer than Sans) ATK: 1 (Karamic Retebution) DEF: 1
Powers: Super Speed, Super Strength, Gaster Blasters, Bones, Blue Bones, Able to grab the soul of his opponent, similar to Sans's ability, can also teleport. All of these require his left eye go glow similar to fanart of Sans's eye.
Weaknesses: 1HP, will wear out after a long, long while.
Physical Description: Blue hoodie similar to Sans's jacket, unzipped to show a white t-shirt, black cargo pants, blue tennis shoes, white socks. His face, body build, and likewise are unknown.
1.Dorian is named after the Assassin's Creed character, Arno Victor Dorian.
2.Dorian's outfit is based off of an outfit The SteelNerfer wore for a Sans costume
Chara Dreemur was the first human that fell into the Underground.
When Frisk fell into the Underground, Chara followed her for most of her journey through the Genocide Run. When Frisk was just about to accept Sans's mercy, Chara possessed Frisk's soul and killed Sans and finished the genocide. After a week of roaming freely, she was confronted by Dorian, and the two of them fought. After two years of fighting Dorian, she found a place where a lot of monsters lived. That place is called Yokai Academy.
Stats: 999 ATK, 999 DEF, LV 99, HP 999
Powers: Super Speed, Super Strength, knows how to use a knife very effectively, can slash to create a red energy attack, similar to how it is seen in GlitchTale.
Weaknesses: Over-confident in her abilities, is evil
Physical Decription: brown hair, red eyes, pink cheeks, green sweater, brown shorts, brown shoes, short
Kurumu Kurono is a female succubus and student at Yokai Academy.