Possible Clues:

• Japanese sneaker?
• Described by 'Assassin in Japan'?
• Practioner of ninjitsu
• At home during January, make up with first three characters from 16 down in Japanese context
• For starters, naturally I'm nimble — Japanese assassin?
• Turtle in a 2014 film
• Assassin retrieves article on James the First going to Ulster
• Martial artist is some man in Japan
• 14th-century Japanese soldier of fortune
• Certain warrior