Eretria doesn't seem like she would be the most important character at first, but she's certainly up in the top five of the series.
She first appears as a "rover" which is a class of man who are thieves with no real homes or loyalty. She uses her beauty and cleverness to get the upper hand on Wil Ohmsford.
She fights with Amberle a good bit before becoming friends. She plays her part to help defeat the Dagda Mor.
She reunites with Wil when the Warlock Lord is about to be revived. Though the two don't seem to reconnect in their previous romance.
Part of which I think was ruined by the creation of the non-book character Mareth. The daughter of Allanon had way more chemistry with Wil in the final season. Though Wil ends up with Eretria in the books and they have kids together.
In the TV show, her romance in the 2nd season is Princess Lyria, which I thought was a decent subplot. Eretria had been known to be bi in the first season, so this one wasn't a shocker. They sort of break up so Lyria can be Queen.
But before the Warlock Lord was defeated she helped the side of good, mainly guided by her mentor (and ex-Druid) Cogline to help.
Cogline helped get the demon out of her body that was causing her to sabotage things in favor of the Warlock Lord.
At the end of the series, she remains with Mareth and Cogline. Though they all learn that Wil is somehow still alive.