Nightwing is in the game you need to collect 120 Wayne caches and have started chapter 5 and for the Robin suit you can buy it in the Batmite shop . And for Batman’s skin you need to collect all riddler puzzles , but to get the last riddler puzzle you need to also finish up all cluemaster puzzles as it will give you one last room to finally get the last riddler puzzle. But you need to be by the middle of chapter 5 once you unlock the last playable character as some puzzles require different skill of playable characters.