There is a side quest available which sends the Dragonborn to talk to Thonar Silver-Blood. Rhiada will greet them, and one can persuade her to let them talk to Thonar. The first dialogue that they pick with him (any of the 3) sets off the 2 undercover NPCs, who will kill Rhiada unless the Dragonborn stops them in time. However, take care to not hurt her or Thonar when in combat against Nana Ildene and Donnel, or guards will appear and the Dragonborn may receive a bounty.
Rhiada is married to Eltrys. If the Dragonborn starts "The Forsworn Conspiracy" quest and talks to Eltrys, they will find out that she is pregnant. However, there is no script of this in the game, as Rhiada does not appear to be pregnant or ever have the baby in the game. It is likely that she may have become pregnant more recently. If she lives through the attack, Thonar will flirt with her, asking about her relationship status.
She is also one of the only characters who owns and wears Tavern Clothes.