Cap is a risk/reward character that requires careful use of assists and patience to offset his faults. While on a technical aspect he is easy to learn, it takes time to pose a threat without leaving yourself open to punishment or kept out by better players. Cap's main issues are his specials and hypers are extremely unsafe, he has no full screen projectile to get in, little to no solo mix up game, and can't compete with superior air zoners that can stay away from charging star and beat out shield slash. Due to these issues, your team and the match up dictate a lot of what Cap can and cannot do.
If Cap is dealing with superior air zoners, his only solo options are to patiently use double jump to bait out a mistake and punish with air shield slash or mount an offense on your way down, or simply evade by using charging star and roll till they come to you. On the ground it's important to attempt to keep them grounded by throwing preemptive shield slashes above them whenever safe and to keep above them and threaten with d+
. Remember Cap only needs to hit once or twice to kill, so be patient and make use of your 1mil+ lifebar till the moment is right.
Cap suffers from many unsafe moves, so using assists to keep you safe is both an essential defensive and offensive options. Charging Star is a great way to get in and harass opponents, but outside of reacting to an opponent for a punish, it is too unsafe to use without an assist. Some assists will let you combo as well as give you free frames on certain versions of charging star and give Cap a way to get in and stay in, making his mid screen game even more threatening. His shields slashes suffer the same and often need an assist to cover him if the shield misses. Lastly his hypers are an amazing tool for punishing opponents, but outside of combos and guaranteed punishes, you take a major risk if your opponent blocks it such as using an X-factor. Having a safe DHC can greatly improve Cap's game by giving him more room to take risks, such as punishing an assist even if the point character might block or countering a counter hyper. Keeping Cap safe is one of the most important things when playing him, if you don't have a team set up to do so, you likely are leaving him without tools he could be abusing like safe charging stars or lock down shield slashes.
Lastly your assists also need to help Cap form an offense and get in. Solo, Cap suffers from a few bad matchups and can be kept out of range to attack or beaten by certain defensive moves that limit Cap's ability to attack, EX: Haggar Lariat or Taskmaster counters. Your assists need to provide a way for Cap to stay within midscreen range and either get in or maintain distance.