Isa's role in the anime is the unpredictable and loyal type, but may turn at any second. As much as everyone loves her, she might be caught up in betrayal if the circumstances are right, but strives to be a loyal partner. Even though she can be violent when emotional, she always ends up following orders in the end, no matter what happens, even with regret.
Another one of her roles is to be a shield for Esther, to protect him in battle even if it may cost her life. She practices her skills daily to make sure that one day she could successfully help him get out of a battle without a scratch.
Isa aspires to become a writer, so she often spends hours hunched over her desk writing in a notebook, and will sometimes fall asleep. This sometimes distracts her or gets in the way of most of her duties, but she eventually makes up for it by working extra hours around the Citadel.
Since her father was a businessman and merchant, she picked up tips from observing him when she was young, so she's a skilled merchant as well. Trading, buying, selling, she can do those things with ease as well, which benefits the group since she sometimes brings back rare or interesting things.