El Pachuco is representative of the very subculture (also called "Pachuco") embodied within this play. A pachuco is a Latino playboy with a strong aesthetic sensibility who wears zoot suits and engages in street gang activity and the nightlife scene. It is also a term associated with those migrating to El Paso, Texas, where the concept of a "pachuco" is thought to have originated. Spanish-speakers would refer to this migration as "going pa' El Chuco."
In the play itself, El Pachuco mirrors Henry "Hank" Reyna, offering himself up as a symbol of self-criticism and negativity. He serves as a "reality check" that forces Henry to give up his destiny, embodying the spirit of Pachuco culture in order to preach fidelity to la Raza.
In a 1988 interview, Valdez, the playwright, said El Pachuco is "the power inside every individual that's greater than any human institution." El Pachuco is Hank's alter ego, the part of him that refuses to allow Anglo society to degrade and abuse Chicanos. He serves as a sort of master of ceremonies, talking to the audience and making comments about the action. El Pachuco is the cool Chicano that everyone wants to be. He is confident and assertive and represents the proud, unwavering Chicano.
Interesting fact, I asked Luis the same question and he told me that he based some of the character of El Pachuco on the Aztec God Tezcatlipoca. El Pachuco's signature red and black outfit were inspired by the colors associated with Tezcatlipoca, and he too like Tezcatlipoca, represents the war and strife in Henry's life. If you wikipedia Tezcatlipoca you'll see a lot of similarities in both Henry and El Pachuco. Hope that helps!