Half of the available main character choices are not available at this time but will be available in the full release of the game. It's easier to just focus on four characters instead of having to do... all of them. The Female Fighter does technically exist and I have art for them, just not available yet.
As for the Striker bug, the issue is likely what weapon you are using. Striker has two weapon skill sets, the Gauntlet and the Pata, and if you have Gauntlet equipped you cannot use Pata Style, so on so forth. If you don't have either equipped, you likely can't use either skill set. If you are using either weapon, pressing I think the side arrow keys or a or d should allow you switch weapon when you're hovering over Attack.
Striker is likely due for an overhaul (and not for the first time...), but let me know if none of these things fixes the problem, cause those are the immediate things that come to mind thinking about how the class works. An update is on the way soon that should fix a lot of issues.