Elijah

"Let's see... patrilineal line... Nineteenth century American midwest. Eighteenth century American Revolution. Sixteenth century Ottoman Empire. Fifteenth century Italian Renaissance! Oh my god! You don't mean...?"

- Álvaro Gramática discovering the boy's identity, 2015.

The son (born c. 2005) of Desmond Miles is a Sage living in New York, though it is unclear if he was aware of his father (and vice-versa) or his Assassin heritage. On 4 October 2015, his mother brought him to an Abstergo clinic in New York, at which point Abstergo discovered he shared the patrilineal lineage of Desmond Miles.

While Álvaro Gramática wished to kidnap the boy immediately and conduct a vivisection on him, Isabelle Ardant opposed this, claiming the young Sage as her own project as well as the immorality of experimenting on a ten year old child. She instead planned to have Abstergo abduct the boy once he was old enough, and then put him into an Animus for the rest of his life so they could study his lineage. However, since Ardant was killed before putting this plan into action, he likely remains free.

Reference

 * Assassin's Creed: Syndicate – Database: Reconstructed Data #010