The AI has two modes, Attack (Offensive) and Cover (Defensive). The default is Cover. If you switch it to Attack, Sheva will become much
more aggressive and start at the top of her weapon list instead of the bottom. I haven't played with it on for long periods, so I cannot comment as to whether that would be a good idea or not. You make the switch by holding the "partner" button and selecting the mode with the D-pad.
If you liked RE4, chances are you will like RE5 also. It plays roughly the same. The game is better on co-op if you have a good friend who has the same play-style as yourself. As far as I've played into the game, the AI has only been a hindrance when I don't want to kill an enemy just yet.