Midnight blade does not ignore defense. What we have is a mis-translation. The actual skill effect for his first standard is to ignore all taunt effects, attacking what is in front of him.

Roshi is not a tank at the start of a match. He need to stack up to actually tank hits.

Roshi is weak to lightning, making him very easy pickings for midnight blade if parked right infront of him.

Kimi on any mood other than +HP is a great way to reduce his ability to tank hits, especially against ninjas like midnight blade, who use tai/nin mixed standards.

Equal power does not mean equal stats. Most dedicated midnight blade users have a highly skewed power distribution, with the main having significantly more power than other ninjas in the lineup.