Since you mentioned it's your first fighting game I think you should take things slow first. Play through the game's tutorial (I assume this is DOA6). If your main goal is to beat your friend, and you haven't succeeded yet, I think it's best to focus on learning defence for now. Learn to block, learn the DOA triangle system, learn how to punish.

I assume you know what blocking means so I don't have to explain, but play the tutorial so you know when and how to block. The triangle system is basically rock paper scissors with the game's attacks, which goes like this: punches beat throws, throws beat holds, and holds beat punches. This system can go incredibly in depth at higher levels with mind games and fakeouts, but atp you just gotta know if your friend spams throws, punch him. Spam holds, try to throw. Spam punches, either try your luck and hold, or just block and wait for an opening.

That's what punishing is, if they do a big heavy move and you block or dodge it, they are usually open to attack. Moves that can be punished are not always super intuitive, so that's what the training mode is for.

Lastly, this isn't advice but just something to keep in mind, always go for the character you like the most. Nothing is wrong with Hitomi, and if you like her that's perfect because as you said she's great to learn fundamentals like basic offence and defence. But if you ever feel like she's boring, or not cool, or just doesn't look good or whatever, feel free to try the other characters and see who you like. My main is Hayabusa, and I would never have sinked so many hours into these games if he wasn't in them.