In-game dialogue (when fighting against Marie) suggests that she fought with Samson against a Skullgirl and/or the Skull Heart in the past, and that Samson was especially fond of her. In Eliza's Story Mode, Samson also reveals that he considers Eliza responsible for Delilah's death due to increased hostility towards Parasites and their hosts, although the exact manner of her death remains unknown.
It has been hinted that, as a host, she was stronger than Filia, as Eliza scorned Samson's current host for being "less worthy" than Delilah while simultaneously remarking that "even [Delilah] wasn't worthy".
Delilah is a Hebrew name that means "delicate" or "languishing", which can be associated with weakness. This also shows in the meaning "[she who] weakened", which stems from how her name, like Samson, relates to the Biblical figure Delilah, a woman who Samson loved but betrayed him.