Cowl may be, or once might have been, a member of the White Council. For example, one hypothesis is that Cowl is the presumed deceased Senior Council member Simon Petrovich which would explain why I said Cowl might have been a member of the White Council. If that is the case, or if Cowl is any other presumed dead member of the Council then Harry won't have the opportunity to meet Cowl at a meeting of current White Council members.
We know very little about Kumori other than she was on a quest to defeat death, the time when she had a one on one conversation with Harry, as part of their truce she took off some kind of bracelet or glove (I forget what the item was, I sure someone can find a description in Dead Beat.) and she was working for, and studying necromancy under Cowl. I believe Kumori studied with Cowl for some time, that she wasn't a recent recruit, because she was quite certain that Cowl had survived many death curses before. I don't think you don't can gain that kind of certainty unless you've seen it demonstrated repeatedly. One more thing, the EMT Lamarr said Kumori is white. He would have seen her hands and probably part of her forearms when she did her holding off death trick with one of Marcone's goons who was shot.
Of course one early and persistent hypothesis is that Kumori is in actuality Elaine. While not unproven I think Elaine is something of a red herring as far as Kumori goes, but even if I am correct, Kumori could be in a similar position vis a vis the White Council as Elaine is. She might not be a member of the White Council. Like Elaine, Kumori may have kept her abilities a secret. In fact I think that's likely or Kumori probably would have been recruited to fight the Red Court.
Harry may presume that Kumori is dead, seeing as Cowl was apparently injured by the Dark Hallow blowing up in his face and Cowl was probably much stronger than Kumori. I think Kumori bailed out and opened a way to escape before the backlash occurred, but currently Harry has no reason to suspect that's the case. Harry was too busy at the time to notice and even Bob presumed Kumori was blasted into atoms.
So what does that leave us with? Well, we have an idea of how tall Cowl is. We know he was badly injured, but the last time Harry saw Cowl was how long ago? How far back are the events in White Night? That wizard healing thing means Cowl might not look quite as beat up as he did in the Raith's Deep. Finally, we know the command word that Cowl used when he threw power at Harry, "Garoosh!" Harry also got a feeling for Cowl's magic. Like Harry's it was touched with darkness, but not blatantly corrupt. I think those last two items are the most important ones. If he meets someone he doesn't know is Cowl, Harry should recognize the use of an identical command word and the feeling of Cowl's magic.
With Kumori, I think she made a big mistake allowing Harry to see the item she removed from her hand or wrist . If Harry sees that item again it should be a huge clue for him. Of course knowing Harry it will take him half a dozen or more chapters to put two and two together.