In War Without End, Part 1, Zathras comes aboard the White Star (stumbling into another Minbari). He sees Sinclair and excitedly approaches. After a bit of back and forth, this exchange happens:
Zathras: Zathras honored to meet you for many reasons. Zathras also honored to meet you [turns to Sheridan] for other reasons.
Sheridan: Uh, such as?
Zathras: [Clicks a sound of disapproval] Oh no, Draal gave Zathras list of things not to say; this was one. [Zathras realizes what he's said] No. Hm, not good. Not supposed to mention One or The One.
As we find out by the end of the two-parter:
Sinclair, Delenn, Sheridan are collectively "The One", being The One Who Was, The One Who Is, and The One That Will Be respectively.
Delenn was present at the beginning of the scene, so why doesn't Zathras react to her in the same way? I suppose it's possible that they have met before, but she says that Draal is "sending an aide" and doesn't refer to him by name, and the look she gives Zathras makes it seem like she doesn't recognize him. So why does Zathras ignore Delenn when she was with Sinclair and Sheridan?