Koga
Who is Koga?
Koga is the leader of the Fuschia City Gym.  He appears in the Ninja-Poke Showdown episode, where Ash battles him for the Soul badge.  He lives in a fantastic Ninja mansion with his sister Aya and their Pokemon, which are of the Poison type (with the exception of some Voltrobs which appear to mainly be exploding security devices.)

Koga's Personality:

He is a reserved, stern person but obviously cares for his family and his pokemon, and in the end treats young trainers with respect. 

Highlights of the episode:

Koga!
Theater Costumes
Psyduck saving the day from Team Rocket
Ash winning the Soul Badge (though it wasn't hard to figure out that would happen)

Koga's Name:

A net search on Koga revealed that there was a whole family or group of Ninja by that name who came from the Koga region of Japan.  There is also a Koga in the amime Ninja Scroll. Koga's Japanese name is
Kyou, which is very cool, but he seems to have ended up with a pretty good dub name.

Koga's Age

I have often heard comments to the effect that Koga is old, but I diagree. Yes, he is a mature man compared to say, Brock, but I wouldn't say he's much over 30 and quite possibly younger.  Also, his sister seems to be not more than 20, even if she is his half-sister an age gap of over 15 years would be rather unusual.

Why We Like Koga:

Beneath that tough Ninja exterior lies a heart capable of kindness, caring, and passion.  (Why else would the Soul badge look like that?)

And he's dead zexy.