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
Is that reported anywhere?
[Edit]
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.