I think it'll be Melgren who's venin. In Fourth Wing Violet joked (before she knew they existed) that she wouldn't be surprised if it turned out that Melgren was a venin (chapter 31), and at the end of Onyx Storm it's revealed that Aaric has the exact same signet as Melgren, just more powerful -- that it said that for each rider's signet there's a venin's signet that matches, like Lilith's and Theophanie's strom wielding