Little is known about Ramia, aside from her treatment of the twins: She heaped love upon Seere, but abused and despised Manah. It was from her that Seere learned the story of the Little Hero, and it was because of her that Manah developed a warped sense of love, which led to her possession by the Watchers. It's alluded in the first game that Ramia and her husband came from a legacy of people skilled in controlling Golems--a skill that she saw in Seere but not in Manah, and therefore despised the "useless" child. In Drakengard 2, the weapon Hero's Knife gives a possible origin for Ramia's relationship with her family.
Regardless, according to Seere, Ramia focused all her anger and hatred into Manah, often beating and berating her. Rather than hating her mother in return, Manah wished to be worthy of the same love Ramia showed Seere, and began to hate Seere instead for being the favorite child. One day, Ramia led Manah out into the wilderness and abandoned her there, where she was later found by the Watchers. The Imperials attack an indeterminate amount of time later and, before dying of her wounds, Ramia forces Seere into a pact with Golem so that he may survive.
Though barely present in the storylines themselves, Ramia has a massive impact on Manah and Seere's characters, and an indirect but important role in setting the events of Drakengard into motion. Her influence can be seen in her children's adult lives as well, as Seere's scholarly habits as hierarch reflect his fondness for his mother's stories, and the relationship between him and Manah remains deeply strained upon their reuniting. When she suffers a mental breakdown halfway through the events of Drakengard 2, Manah reverts to a childlike mentality in which she often cries out for her mother and begs not to be beaten. The Watchers seem to aggravate and feed off of these feelings as a source of their power over Manah, so that when Nowe purges the possession from her, her trauma seems to fade with them.
In the events of Shi ni Itaru Aka, it's unknown as to why Ramia wasn't killed along with One's other descendants.