Shizuo’s playstyle can differ depending on the assist you pick up for him, you can setup for command grab unblockable setups with Iriya, go for a command grab mindgame gameplan with a counter assist or go with something more combo focused with Dokuro.
Iriya is the best assist in the game and offers so much for just about every character in the cast, Shizuo is one of those characters. By carefully applying Iriya 5S and building up 3 bowls for the air unblockable fireworks you can setup a difficult situation for your opponent to escape from via his command grab.
Even outside of this nasty setup his utility with her assists give Shizuo a lot to work with for combos, oki and defense options all of which are things Shizuo wants.
Izaya is Shizuo’s canon partner and is go to counter assist of choice, Izaya not only offers extended combos through his 6S (which you don’t really need to use most of the time), the greatest thing he offers is his 5S which is a projectile counter. It creates nasty situations where you opponent has to guess between staying grounded to avoid an anti command grab or jump to avoid the grounded command grab. It’s a 50/50 guess and that’s what a counter assist like Izaya offers to Shizuo.
Alternatively there are other counter assist options, all ranging in different strengths. Feel free to experiment and see which one you feel most at home with. The other counter assists are Touma and Zero.
If you’re looking for something more combo focused, giving Shizuo easier air to ground combos that he lacks with other assists and within his tool kit then Dokuro might be the assist you want to go with.
While you sacrifice having any sort of command grab mental gameplan as you would with a counter assist you do get a lot more tools to give Shizuo to make him more well rounded against the cast. 5S well give you a form of “anti air” to snipe antsy jump ins and 6S is a strong neutral tool that beats your normal projectile.
Celty is a hybrid assist who can give Shizuo neutral tools in Celty 5S that help give Shizuo more neutral control as well as give him assistance in his oki with 6S. While his oki would probably be stronger with a counter assist it’s more of a preference with how you want play.