Y button - Evasion roll. Use this roll to avoid quick one hit attacks or to get out of the way of some of the enemy bullets. Extremely useful maneuver. In Spawn, staying in one place for more than a quick second can get you killed in a hurry. Attack up greatly increases your attacking power, and for some fights, it is a necessity due to some of the bosses' LONG energy meters.
Defense is perhaps even more important than attack.
Some of your opponents are capable of killing you in one hit, quickly draining your time to finish the level. Obtaining defense allows you to survive, and find some health. Speed is useful for quickly getting around some of the opponents attacks. Ties in with knowing your enemy, if you have a fast, but weak character, do not try to fight a head on fight. The time spent taking them out is far better spent than the time taken away after you are killed.