Ninjas and rogues fill similar archetypes in fiction; they're both the stealthy sort who rely on trickery and agility instead of armor and power. Honestly, Froakie being associated with a rogue makes it more likely the last evolution will look like a ninja.
Indeed. Ninjas are sometimes depicted with scarves, including a certain slug and Frog
One of the pokemon I regret not being able to use in B/W. Stupid trade Evos.