Ianthe's father, the poet Percy Bysshe Shelley, eloped with very young Harriet and then left her when she was pregnant with Ianthe, for sixteen year old Mary. Mary Shelley was the author of "Frankenstein".

See "The Complete Poetry of Percy Bysshe Shelley, Volume 2" By Percy Bysshe Shelley - there is a notation on Google Books which refers to the "Esdaile Notebook", a collection of poems by Shelley which passed down from Harriet to her daughter Ianthe....it continued to be passed on through the Esdaile until 1962 when it was sold by auction. This collection was purportedly meant to be a copy of his poems, in case the originals went astray, and were written casuallyt without punctuation. However, he did write in the Notebook two sonnets - one to Harriet, and one to Ianthe - both finished with punctuation.

https://www.findagrave.com/memorial/205200114/eliza_ianthe_esdaile