Even she actually doesn't get that much screen time—and when we do see her, her eyes are usually swimming with tears—Lucilla actually plays super-important role in the plot of Gladiator.
Thankfully, right? If she had to be 110% miserable throughout this film and didn't even forward the storyline, we'd feel awful for her.
For most of the film, she's the very unhappy (and very sexually harassed) sister to her evil brother Commodus. There's even some doubt about just how complicit she is in her brother's machinations—Maximus screams at her the first time she visits him in his gladiator barracks in Rome.
But it turns out she isn't that complicit. In fact, it turns out that she carries the impossibly heavy burden of knowing just what a slimy scumbag her brother is…and having to pretend to be on his side. We have to hand it to her: she's a good actress, saying things like:
LUCILLA: The mob is fickle, brother. He'll be forgotten in a month.
We imagine that line must have stuck in her throat a bit. She isn't blind, and it's obvious that a) the mob isn't fickle and b) Maximus' rep isn't going to be forgotten any time soon. But when you have a psycho brother who will kill—or French kiss—you if you cross him, you say what you need to in order to stay alive.
Besides just wanting to stay alive, however, Lucilla also protects herself because, well, she wants to save Rome from her evil brother. In a lot of ways, she's trying to do what Maximus cannot do while he's enslaved: fulfill her father's dying wishes.
She puts together the meeting between Gracchus and Maximus, and she's the one who lets Maximus know they have to act quickly because Commodus has arrested Gracchus and is getting closer to uncovering the plot.
Even though Lucilla's the linchpin of the plan, however, she's also its undoing. Her son Lucius lets something slip (while playing he refers to Maximus as the "savior of Rome," that's all Commodus needs), so she must have said things where he could hear.
As a result, Lucilla reveals the whole plan to Commodus (offscreen) because he says, in no uncertain terms, that he'll kill her son if she betrays him again. Oh, and he'll also kill her son if she kills herself…or looks at him the wrong way. Bonus: he also demands that she love him the way he wants and gets preggo with his baby.
Lucilla's a mostly noble character, and one who, despite being closely watched by her brother, does her best in a bad situation. We can't fault her for coming clean with Commodus (he would have killed her son if she hadn't), but we can fault her for somehow letting Lucius hear things he shouldn't have.
Wasn't there an Ancient Roman equivalent of telling your kid, "Earmuffs!"