Sigma

Sigma is a “vanilla” operator that provides a non-cancellable 20% barrier. Where she was once a dominant PVP operator, she has now been sidelined in favor of more impactful operators.

In PVP, she is a defensive PVP operator for when you are unable to take advantage of Olivie. Sigma provides zero utility beyond that which is inherent to barriers (damage that exceeds a barrier's max health will not bleed over into your unit's actual health). In practice, only Evolved-1 comps can take full advantage of Sigma. Every other comp would sooner use someone else. Sigma’s operator trigger order can also be awkward. Strikers are not always easy to fit into comps, and because the week’s meta may be sniper dominated (e.g. Awakened Rosaria, Curian), there may be better non-striker frontline options, hence making it awkard to find an appropriate time to deploy a striker frontline without stacking frontline tanks. Bringing Ifrit means running a mediocre DPS unit that struggles to keep up with other damage dealers.

Her 50% Role ADV DMG INC buff is largely irrelevant, coming out to a ~10% overall damage increase against applicable targets.

In PVE, she sees some usage in content where survivability may be an issue, or in content where you cannot use duplicate operators across multiple teams (like Dimension Trimming), but other than that, she is really not necessary at all.