If you don't already have a ninja, Saki would be a good choice to pick up in order to help with maps that have a ban on ranged units since melee support can still be played. Although, a ninja becomes less essential in the future as more melee support units are added, but until then, ninjas are our only option.
Additionally, Saki herself is a rather solid unit, and become even better with her Awakening and Skill Awakening, increasing her attack speed and allowing her to attack up to three enemies respectively.
As for Akane, no she isn't really important or essential for future missions. Samurai become more viable once they're Awakened since they can block and attack three units at once. Akane is decent since she can dodge with her skill, although its effectiveness wanes as she blocks more units, and can allow for some niche strategies (although certain other samurai such as Shizuka can perform even more overall), but can be considered a unit for completionists more than anything else.
