Mary Coleman (Eliza's aunt--see S26n2 ) or Eliza herself.

If the latter, Louisa's formality is perhaps explained by a desire to give her cousin treatment egual to that accorded the other members of the party. Why Sally Allen has not been accorded the same treatment could be explained by the speculation that the idea to refer to Eliza as "Miss Coleman" has come to Louisa only after she has written "Miss Macy" and realized she would be using the title "Miss" for two of the next three young ladies. Or perhaps, if "Miss Coleman" is Eliza, Louisa is just being playful. (See also O19n4 -and, for another instance of Louisa's formality with a very close friend, N5n3 .)