This word is credited with developing in both Italian and Spanish, most likely from each language’s word for “sympathetic.” A simpatico person can be simply likable, but a more specific usage relates to compatibility. Your simpatico friend has shared interests — perhaps you met at a Comic-Con and bonded over your love of early Star Trek memorabilia. They could be curmudgeonly, but as long as you have that shared bond, they are still considered simpatico.