Warning,Spoilers both for Coraline and for Undertale,and for some aspects/parts of the fanfic.
Coratale is a DeviantART Fanfic who takes place after the events of the movie "Coraline" and mixes the universes of Undertale and Coraline. Coraline decides to fall into the Underground to save the SOULs from the possible humans trapped down,unaware that her old enemy,The Other Mother,is still alive and chasing her down.
Coraline is the main protagonist,and has her devolped personality she got with the events of "Coraline" and the clothing she recieved from The Other Mother while she was tricked to stay in the Other World (blue starry shirt,black pants and blue boots). She falls on the Underground after learning about it thanks to her neighbors,and decides to go into a quest to save the SOULs trapped there just like she did in the Other World with the Ghost Children.
Caroline is the first fallen human and,presumably,the adoptive daughter of the Dreemurr family and Asriel's sister and best friend.She appears to Coraline on the First Chapter,showing she's very similar to the protagonist,but with a green shirt with yellow stars,brown pants and red boots,and a paler skin tone and pink cheeks. She also displays a playful and snarky personality upon chatting with Coraline.
Flowey is one of the main antagonists,being Coraline's new enemy trough her journey on the Underground. He attempts to trick Coraline just like in the game Undertale,but his mask almost falls when Coraline mentions Caroline,showing Flowey is actually Asriel but in a reincarnated,"fallen" form. He also seems to know The Other Mother in the Fifth Chapter,and fights agains't her for the SOUL of Coraline.
Toriel is a protagonist,and helps to guide Coraline on the Underground. She is the same motherly but strict figure she was in Undertale,but openly tells to Coraline she actually though she was Caroline,her adoptive daughter,because of the similar appearences. She wishes good luck for Coraline before leaving her free to go the whole Underground.
The Cat was one of the greatest allies of Coraline on the previous journey trough the Other World,and reappears to help her on the Underground. He seems to be aware of the Underground,the Monsters and of Toriel,but claimed to Coraline indirectly he was busy keeping an eye on The Other Mother to visit them. He didn't follow Coraline until Snowdin,but off-screen,he warned Sans about her.
Patricia is the second fallen human,who Coraline meeted upon joining her weaponry (Toy Knife,Faded Ribbon).Patricia was a orphan,wishing patiently for someone to adopt her,but decided to explore the world to find her family. She falled on the Underground,was attacked by Flowey but saved by Toriel. Patricia made a journey on the Underground and meeted her fate at the hands of Asgore. After this,she waited for someone to set her free,but all the Humans died until Coraline came.
The Other Mother
The Other Mother is one of the main antagonists,being Coraline's old enemy trough her journey on the Underground. Apparently,she could find a way to exit her world,The Other World,and followed Coraline to the Underground to get a revenge for being defeated. She seems to know about Flowey and even his true identity (she calls him "Prince Dreemurr") in the Fifth Chapter,and fights agains't him for the SOUL of Coraline,her old enemy.
Sans & Papyrus
Sans and Papyrus are protagonists,and sentries on Snowdin.
Sans is mostly like his original Undertale appearence (lazy,pun-lover,prankster),but thanks to the circumstances and of Coraline's still-present arrogance,he displays his more serious side more often.
Papyrus is also similar to his Undertale appearence,and even claims that the Lamp "had of Sans' puns" even trough it was Coraline.