Koga is the leader of the Fuschia City gym, just like how he is in the original Pokemon Red and Blue. I remember him well in the anime, but I will say he looks a little younger in this remake. I like how he is a ninja...I guess he's one.
His gym is a bit tough to navigate. There' fog and invisible walls. Which makes it a bit tricky but not terrible difficult to get through.
Once you face him, he's got a team of mostly poisonous pokemon. Which despite your pokemon power levels, you'll need to be careful.
But since I knew what I was getting into, he wasn't too bad. Charizard really helped me out here.
And with it you get the Soul Badge, and less than half of Kanto's gym leaders await you now.