[quote=@Prostagma]
You know, [@samreaper], you could always just ignore Devil Fruits entirely and make your dude a chemist, shouldn't be too far of a stretch since he's already a doctor apparently. Remember, you can do weird stuff in One Piece without needing a Devil Fruit; you could very well say he received his abilities from chemical treatments. You can do the pain fluid bit by saying his pores produce a transdermal irritant- in other words, sweating tear gas. Shutting down his own pain response (which is generally a really bad thing just saying) can easily be explained with anesthetics I guess. 

Now, the turning intangible and mental control things would be kind of hard to justify, so maybe let's run with the drugs and chemicals theme. Like, say, he's got a spinal tap with a variation of the Energy Steroids that he can use to temporarily bulk himself up ala Bane, or possibly some sort of gauntlet apparatus that can store up his sweat and launch it like Bakugou from MHA. Maybe he's got sort of a mad doctor thing going, mixing up crazy new drugs and steroids to test-run on his crewmates; that could be a simple way to have your character help out and give people a valid reason to keep him around, yeah? Just some suggestions, is all.
[/quote]

Or how about this; Make him the Medic from TF2.  Syringe gun that shoots out chemical cocktails and all that. you gotta remove the intagiblity and mental control things, but they're the things people are complaining about anyway.

Plus who wouldn't want a crazy experimental German guy on their side. You can live Meet the Medic!

https://www.youtube.com/watch?v=36lSzUMBJnc