Hm, I'll try to give my 2 cents real quick
Chelsia has some good advantages, 1, she has a sizeable AP advantage, of about 3x, maybe a bit less then that, but still noticeable, her LS is also about 3.7x, a pretty great advantage but not as big, probably won't make too much of a difference considering Chelsia normally punches while The Heartlest One seems to use more ranged attack stuff. Rangewise they are about the same and the arena facors Chelsia, as she is a much better acrobat then The Heartless One, probably allowing her to move a bit more erratic then THO is used to
Ultimately, The Heartless One has a few things going for him, it being a Hefty Immortality type 2, Status Effects and a possible Soul Manipulation. Bleed seems to be something that might make a difference when coupled with THO's Type 2, while the Evasion Debuffs would help against Chelsia's Acrobatics and I don't really know what the Light Sensitivity does. Overall, it would be a tough match for Chelsia already but there is also that Soul Manipulation, if it really is Soul Manip, then Chelsia just loses and it might be a stomp, with it out of the question, I think Chelsia has this in a very high-diff fight, mainly because the profile for the Heartless One doesn't really tell me how he can apply said Status Effects
So, TLDR: With Soul Manip, this is a stomp for THO, without it, I think Chelsia takes it unless more information is said about THO's powers