That matchup is one of the ones that I think can be difficult to just give a flat number to in a game like Skullgirls.
I think it depends almost entirely on the situation in which the 2 begin from.
For example, at round start, Bella just absolutely and completely dominates Painwheel, Painwheel either has to take a ridiculous risk or concede almost a full screen of space. This puts Bella in such a powerful position that Painwheel hardly has the tools to escape, making the matchup seem very much in Bella's favour.
If Painwheel is coming in off a DHC into hatred install or Bella incoming after a snap/kill. Painwheel has enough advantage to create some space for nails, or to begin pressuring Bella. Pressuring Bella is sometimes questionable and Bella certainly has tools to escape that situation, but a good bait from that scenario, or safe pressure allows Painwheel to keep momentum. From range, Painwheel has basically no reason to go in and Bella struggles tremendously trying to get in on a Painwheel especially assisted, as long as Painwheel doesn't corner herself I think she wins from this position.
Overall I'd say Bella has more advantages in the matchup than Painwheel, but its hard to put a straight up mark on this one, just because it can swing so heavily depending on how the matchup begins.