I honestly don't think Quentin knew what to do with him.
IIRC correctly, DeNiro wanted the Max Cherry part, but QT had his heart set on Forster. So he gave him Louis as a consolation prize, or something like that.
Looking back at it, I think it was probably for the best. The movie is essentially a love letter to Pam Grier; it's
her movie,
her name in the title,
her character that drives the plot. Forster as Max
works because Forster is subdued in performance and was relatively obscure enough at the time to allow Grier's performance to dominate. You place someone like DeNiro apart from her, and it upends the whole film and totally destroys the dynamic between Max and Jackie. If he were placed in the Cherry part, I don't think he as an actor would allow himself (consciously or not) to be dominated by Pam Grier.