Sabrina is the leader of the Saffron City gym, which is one of the latter gyms in the game. I remember her well in the anime. They had a neat story-line where she had some pretty creepy psychic powers. She doesn't have much of a story here though, just like the original Red/Blue games for which this is a remake of.
Getting to her is the hard part. You have a bunch of tall skyscrapers (how is that a gym?) and you teleport around until you get to her. If this wasn't a RPG where you need all the EXP you can get, it would make sense to find the better path. But this is rather annoying.
Anyway, she mostly has a bunch of psychic type Pokemon, but some like Slowbro are also hybrids. Meaning you don't need a pokemon with moves that are weak to just psychic alone.
But her Pokemon can pack a serious punch. I knew she'd be challenging, so I planned it all out. I got a few painful hits from her team, but I managed to defeat her pokemon with room to breath and more.
When you defeat her, you get the Marsh badge. Meaning you can trade Pokemon to beat the game with ease, if you know anybody cool that is.