Very clever and very crafty. First we think Harry Potter confectionery, then I was searching for names that sound like 'Ribbit.'
Tricksy Huinesoron! Tricksy in his untricksiness!
FATE OF HERO? (Notholding out much hope with this, but ....)
Maybe there are 5 pairs, each a hero(ine) with their fate. Frodo and Eressea and Elessar and Tomb are there already.
In which case, could the last two be Mithrandir and Balrog, with something like mound or hill to go with Arwen and maybe Denethor to go with fire?
Mithrandir has MIR in it (and does rest of name have the elvish for dark?) and we use the 5th letter of Balrog - did it roar when it trapped its wing?