Below is a piece by Eliot where he is reminiscing about his student days in Paris:
I am willing to admit that my own retrospect is touched by a sentimental sunset, the memory of a friend coming across the Luxembourg Gardens in the late afternoon, waving a branch of lilac, a friend who was later (so far as I could find out) to be mixed with the mud of Gallipoli.
The hyacinth is a male symbol (see myth of Hyacinthus)
The Exploring The Waste Land site has a poetic translation of Ovid's Hyacinthus transform'd into a Flower from Metamorphoses, Book X. It was
Translated into English verse under the direction of Sir Samuel Garth by John Dryden, Alexander Pope, Joseph Addison, William Congreve and other eminent hands.
The lilac also is a male symbol (due to its form). Thus we have a connection from these lines, where Eliot remembers a sublime moment with a friend in the hyacinth garden, to lines 1 through 3, where Eliot laments the lilac season which reminds him of the loss of this friend. We are brought from lamentation to ecstatic remembrance and then back to lamentation again immediately in line 42 ("Desolate and empty the sea").
But other than the symbolism of the hyacinth is there any reason that we should believe that Eliot is remembering a male friend? We have the following to lead us to this conclusion:
Maybe in reading "hyacinth girl" we should put the emphasis on "girl" instead of "hyacinth" ("They called me the hyacinth girl").