What's so special about leaders?
What ability do they have?

After maxing out the Leader (5 stars) each Leader gets an ability that the entire team can use. Some leaders increase your pill count, some increase the amount of experience your other cards get after winning a fight, and others increase your health.

Is Ashigaru like the least wanted Leader?

They are all good in their own way. depends on how you want to stategixe your attack.

I have maxed out Ambre and Ashigaru....level 5...I dont know what these do yet..but...Ambre has (P)Team:Courage 2.....and Ashigaru has (P)Strike Back

Team: Courage 2 means you'll get a increase of 2 powers for any of your
teammate if it's the first to strike in a round.
Strike Back: means you'll always wait for your opponent in the first round
of the match.
All powers will get a nice description on the website very soon.

Ty Fraggle