ADK

ADK (Alpha Denshi Kabushiki kaisha¹⁾) is a Japanese videogame company. They began as an electronics company in 1980, soon moving to arcade game development, and eventually becoming the first company to make games for the Neo-Geo. I'm unsure to what extent they were involved with SNK—whether, or not, they were a second-party studio—but their properties are now owned by SNK Playmore.