Light

Light does no damage and provides no healing. It applies a flat damage reduction for a few rounds, plus one round of immunity to stuns. The shield can never cover its whole cooldown - there is always a round where you are exposed - so it is a defensive window to play around rather than a permanent buff. Averaged over a fight, a fully invested legendary is worth around a third of incoming damage.

Light is cap-bound by design: both the tree and star rank push toward a ceiling rather than past it. Skill points that would exceed the shield's maximum duration are refunded as extra damage reduction, and light folds its proc investment into mitigation because it never reads proc chance.

Spike damage and boss fights with heavy control. Light is the only pet that answers stun-locking, and the only one whose value does not depend on the fight going a particular way.