Eh, it depends on the season, if it's a season that has descendants of ancient Chinese warriors, beings that look humans, guardian warriors from the dinosaur age, & I guess people from the future, then who cares, now if it's set in the year the sentai is made & they have regular people & they have excuses for lack of last names, who cares, but if they don't well, it's gonna be kinda strange but I'll still watch it, there's my 2 cents :lol.

Unless their last name is plot relevant like in Timeranger or Shinkenger I honestly don't care... Though considering how I personally take the time to give each character that I create both a first and last name I'm not sure how credible my opinion is. :lol