“Grump with a heart of gold” is a classic, but also consider:

• Mercenary scumbag who invariably manages to contrive a hilariously self-serving justification for doing the right thing
   
  
• Seeming fatalist who in fact cares a great deal, but unconvincingly maintains they’re only getting involved because they have nothing better to do with their time
   
  
• Fussy coward who insists on facing every danger headlong because they’d rather keep the peril where they can see it
   
  
• Repellent jerk who’s actually very principled, but takes great pains to conceal it because they’re inexplicably committed to upholding their reputation as a total sleazeball
   
  
• Stoic utilitarian whose logic is always just far enough aslant that you’re never 100% sure whether they’re fucking with you