Symbolism, Imagery, Allegory
Rojack feels so powerful when he has Shago's umbrella that he might as well be holding Excalibur.
After all, Shago looks awfully intimidating when he barges into Cherry's apartment, carrying the "furled umbrella, taut as a sword in its case" (7.9). Of course, Rojack doesn't yet realize that Shago is actually a big softie, trying to validate his own masculinity by intimidating Rojack—if anything, Shago brings the umbrella with him because it makes him feel tough, not because he is tough. It's a prop for something he feels he lacks.
Rojack doesn't experience the power of the umbrella himself until he visits Kelly. After getting fed up with Kelly's manipulation, Rojack grabs the umbrella and feels "stronger now, like a derelict provided with a cigarette, a drink, and a knife" (8.333). He goes on to smack Kelly in the face with the umbrella a few moments later, emphasizing the newfound confidence it instills. Again, then, we see the umbrella acting as a crutch against insecurity.
It might not be as cool as a lightsaber, but Shago's umbrella gets the job done. Well, for Rojack, anyway.