There largest customer is Citadel, who now have a big position in Melvin which is one of the hedgefund's with the large position that started it all but no collusion to see here
Answered my own question with a quick google - Citadel Securities executes in the market on behalf of Robinhood. So not ownership, but a definite interest.
Hmm I googled it before I wrote it and thought I confirmed it but it looks like not partial ownership but Citadel is there largest customer, so updated my previous post to reflect that
There largest customer is Citadel, who now have a big position in Melvin which is one of the hedgefund's with the large position that started it all but no collusion to see here