Now for my thoughts on the match-up.
ERROR Sans: Due to timelines pretty much being synonymous to (or perhaps, dare I say, lesser than) universes in Undertale, Celica has apt enough defence to resist his Timeline Destruction. KARMA doesn't seem to have much effect, as even though she has a True Neutral aliment, she demonstrates no malice or impure intent, which KARMA is based around attacking. His Telekinesis, Teleportation and Gravity Manipulation will also be useful, and his range is about the same as hers (if not a little better?) via Teleportation, despite what his profile says. His biggest edge, however, is his Soul Manip and ability to attack souls, as (aforementioned) Celica lacks Astral Defence.
Celica: Her Space-Time Manipulation is superior to ERROR Sans' Spatial Manipulation, and their Reality Warping should counter each other, so she has some good counters to some of his otherwise powerful abilities. She could counter his Soul Manip to an extent with her Spiritual Force Manip, but this isn't likely enough to completely counter ERROR Sans' edge. I previously said that ERROR Sans' would aim to kill her, but her Type 4 Immortality would actually hinder this plan, forcing him to attack her soul, which... is both good and bad for her. Abilities in her favour would be Penance Stare (to distract him), Evil Eye (which can counter paralysis via Telekinesis or Gravity Manip, say), Mind Manip and Dream Manip.
To compare the two, ERROR Sans is going to be more than likely attacking with his more useful abilities to get her soul, but her Evil Eye and Penance Stare would help delay potentially harmful blows, and her Mind Manip could actually subdue him, making him no longer wishing to harm her. Her Dream Manip could be used for all sorts of things, perhaps granting her soul a shield (which is possible in UT logic), or summoning her own weapons (perhaps copying his Gaster Blasters) to attack him. So... I think in the end, despite the close call, Celica could win this.
